Always 0 when evaluate Objective-C Enum value of Sketch on MacOS10.15Beta
Hello, I have found a problem; but I don't know why.
 
from MacOS10.15Beta,
SketchApp Run Script:
```
const r = NSUTF8StringEncoding

console.log(r)
```
The result always 0, no matter what Objective-C Enum value here;

But MacOS10.14, MacOS10.13, is OK;
when this case, value is 4, 4 is correct from documentation of Xcode.

So I have to manually replace all Objective-C enum value to integer,
in order to make plugin works on MacOS10.15Beta; that's not so good.

