JSON. stringify crash
i have a symbolInstance and change it into a nativeObject, my sketch is 52.6
```
var SymbolInstance = require('sketch/dom').SymbolInstance;
var SymbolInstance.fromNative(instance);
JSON.stringify(instance);
```

the code crash
```
rror: Cannot create a gradient from a MSImmutableGradient
line: 1
sourceURL: /Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js
column: 26438
stack: value@/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:26438
/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2142
forEach@[native code]
value@/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2102
map@[native code]
/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2176
forEach@[native code]
value@/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2102
/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2286
forEach@[native code]
value@/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2102
/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2286
forEach@[native code]
value@/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2102
map@[native code]
/Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js:1:2176
forEach@[native code<…>
```