Invalid fontWeight causes Sketch to crash
I am reporting a bug.

**Observed behavior:**

Crash

**How to reproduce:**

Passing an invalid `fontWeight` to `<Text>`, ex. `"Regular"`.

**Sketch version:** 51

**Stack Trace:**

<details><summary>View Trace</summary>

```text
0   com.apple.CoreFoundation      	0x00007fff568be522 CFRetain + 114
1   com.apple.CoreText            	0x00007fff584515a9 CTFontDescriptorCopyNameForSystemFontOfWeight + 201
2   com.apple.UIFoundation        	0x00007fff7ab2bf35 __NSGetSystemFontVariants + 77
3   com.apple.UIFoundation        	0x00007fff7aa978d7 +[NSFont systemFontOfSize:weight:] + 39
4   libffi.dylib                  	0x00007fff7d5a6f64 ffi_call_unix64 + 76
5   libffi.dylib                  	0x00007fff7d5a777c ffi_call + 808
6   com.cocoascript.CocoaScript   	0x0000000109225d35 _MOFunctionInvoke + 4706
7   com.cocoascript.CocoaScript   	0x00000001092268cb MOFunctionInvoke + 88
8   com.cocoascript.CocoaScript   	0x0000000109200567 MOFunction_callAsFunction + 126
9   com.apple.JavaScriptCore      	0x00007fff59b74f86 JSC::JSCallbackObject<JSC::JSDestructibleObject>::call(JSC::ExecState*) + 630
10  com.apple.JavaScriptCore      	0x00007fff599b5445 JSC::LLInt::setUpCall(JSC::ExecState*, JSC::Instruction*, JSC::CodeSpecializationKind, JSC::JSValue, JSC::LLIntCallLinkInfo*) + 837
11  com.apple.JavaScriptCore      	0x00007fff59b48ee9 llint_entry + 29223
12  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
13  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
14  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
15  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
16  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
17  com.apple.JavaScriptCore      	0x00007fff59b49357 llint_entry + 30357
18  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
19  com.apple.JavaScriptCore      	0x00007fff59b49357 llint_entry + 30357
20  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
21  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
22  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
23  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
24  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
25  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
26  ???                           	0x000029b1bbe8cda4 0 + 45843338546596
27  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
28  ???                           	0x000029b1bbe0a501 0 + 45843338011905
29  ???                           	0x000029b1bbe6d3df 0 + 45843338417119
30  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
31  com.apple.JavaScriptCore      	0x00007fff59b48e78 llint_entry + 29110
32  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
33  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
34  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
35  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
36  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
37  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
38  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
39  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
40  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
41  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
42  com.apple.JavaScriptCore      	0x00007fff59b48ef7 llint_entry + 29237
43  com.apple.JavaScriptCore      	0x00007fff59b41ada vmEntryToJavaScript + 304
44  com.apple.JavaScriptCore      	0x00007fff5a1a0063 JSC::JITCode::execute(JSC::VM*, JSC::ProtoCallFrame*) + 147
45  com.apple.JavaScriptCore      	0x00007fff599b7ea4 JSC::Interpreter::executeCall(JSC::ExecState*, JSC::JSObject*, JSC::CallType, JSC::CallData const&, JSC::JSValue, JSC::ArgList const&) + 548
46  com.apple.JavaScriptCore      	0x00007fff599b7c6e JSC::call(JSC::ExecState*, JSC::JSValue, JSC::CallType, JSC::CallData const&, JSC::JSValue, JSC::ArgList const&) + 62
47  com.apple.JavaScriptCore      	0x00007fff59af2df4 JSC::boundFunctionCall(JSC::ExecState*) + 692
48  com.apple.JavaScriptCore      	0x00007fff59b41c54 vmEntryToNative + 310
49  com.apple.JavaScriptCore      	0x00007fff599b7ee2 JSC::Interpreter::executeCall(JSC::ExecState*, JSC::JSObject*, JSC::CallType, JSC::CallData const&, JSC::JSValue, JSC::ArgList const&) + 610
50  com.apple.JavaScriptCore      	0x00007fff5a2f0db2 JSC::profiledCall(JSC::ExecState*, JSC::ProfilingReason, JSC::JSValue, JSC::CallType, JSC::CallData const&, JSC::JSValue, JSC::ArgList const&) + 178
51  com.apple.JavaScriptCore      	0x00007fff599b7b06 JSObjectCallAsFunction + 502
52  com.cocoascript.CocoaScript   	0x0000000109201d79 -[Mocha callJSFunction:withArgumentsInArray:] + 307
53  com.cocoascript.CocoaScript   	0x0000000109201c2a -[Mocha callJSFunctionWithName:withArgumentsInArray:] + 100
54  com.cocoascript.CocoaScript   	0x0000000109201ad7 -[Mocha callFunctionWithName:withArgumentsInArray:] + 55
55  com.cocoascript.CocoaScript   	0x00000001092071ef -[COScript callFunctionNamed:withArguments:] + 97
56  com.bohemiancoding.sketch3    	0x0000000108bdeb1f 0x1088f1000 + 3070751
57  com.bohemiancoding.sketch3    	0x0000000108bdf0c2 0x1088f1000 + 3072194
58  com.bohemiancoding.sketch3    	0x0000000108bdeeae 0x1088f1000 + 3071662
59  com.bohemiancoding.sketch3    	0x0000000108a7cc80 0x1088f1000 + 1621120
60  com.bohemiancoding.sketch3    	0x0000000108c81de3 0x1088f1000 + 3739107
61  com.bohemiancoding.sketch3    	0x0000000108a7c702 0x1088f1000 + 1619714
62  com.bohemiancoding.sketch3    	0x0000000108a7d7b1 0x1088f1000 + 1623985
63  com.apple.CoreFoundation      	0x00007fff56939c3c __invoking___ + 140
64  com.apple.CoreFoundation      	0x00007fff56939b10 -[NSInvocation invoke] + 320
65  com.apple.CoreFoundation      	0x00007fff56952c76 -[NSInvocation invokeWithTarget:] + 54
66  com.apple.CoreFoundation      	0x00007fff569385ac ___forwarding___ + 748
67  com.apple.CoreFoundation      	0x00007fff56938238 _CF_forwarding_prep_0 + 120
68  com.apple.CoreFoundation      	0x00007fff56939c3c __invoking___ + 140
69  com.apple.CoreFoundation      	0x00007fff56939b10 -[NSInvocation invoke] + 320
70  com.apple.Foundation          	0x00007fff58ae6a06 -[NSConnection dispatchInvocation:] + 136
71  com.apple.Foundation          	0x00007fff58ae5d17 -[NSConnection handleRequest:sequence:] + 1461
72  com.apple.Foundation          	0x00007fff58a9c7ad -[NSConnection handlePortCoder:] + 1152
73  com.apple.Foundation          	0x00007fff58a9bf2c -[NSConnection dispatchWithComponents:] + 53
74  com.apple.Foundation          	0x00007fff58a9b9b4 __NSFireMachPort + 253
75  com.apple.CoreFoundation      	0x00007fff569455bb __CFMachPortPerform + 347
76  com.apple.CoreFoundation      	0x00007fff56945449 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 41
77  com.apple.CoreFoundation      	0x00007fff56945395 __CFRunLoopDoSource1 + 533
78  com.apple.CoreFoundation      	0x00007fff5693cf50 __CFRunLoopRun + 2848
79  com.apple.CoreFoundation      	0x00007fff5693c1a3 CFRunLoopRunSpecific + 483
80  com.apple.HIToolbox           	0x00007fff55c24d96 RunCurrentEventLoopInMode + 286
81  com.apple.HIToolbox           	0x00007fff55c24b06 ReceiveNextEventCommon + 613
82  com.apple.HIToolbox           	0x00007fff55c24884 _BlockUntilNextEventMatchingListInModeWithFilter + 64
83  com.apple.AppKit              	0x00007fff53ed7a73 _DPSNextEvent + 2085
84  com.apple.AppKit              	0x00007fff5466de34 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
85  com.apple.AppKit              	0x00007fff53ecc885 -[NSApplication run] + 764
86  com.apple.AppKit              	0x00007fff53e9ba72 NSApplicationMain + 804
87  libdyld.dylib                 	0x00007fff7ec56015 start + 1
```

</details>