Cannot read property 'map' of null

I get this error :
```
C:\Users\user\project\node_modules\shifty\dist\shifty.node.js:1091
Uncaught TypeError: Cannot read property 'map' of null
at getFormatChunksFrom (C:\Users\user\project\node_modules\shifty\dist\shifty.node.js:1091)
at getFormatSignatures (C:\Users\user\project\node_modules\shifty\dist\shifty.node.js:1281)
at tweenCreated (C:\Users\user\project\node_modules\shifty\dist\shifty.node.js:1440)
at Tweenable._applyFilter (C:\Users\user\project\node_modules\shifty\dist\shifty.node.js:588)
at Module.interpolate (C:\Users\user\project\node_modules\shifty\dist\shifty.node.js:1538)
at Path._calculateFrom (C:\Users\user\project\node_modules\progressbar.js\src\path.js:152)
at Path._resolveFromAndTo (C:\Users\user\project\node_modules\progressbar.js\src\path.js:145)
at Path.animate (C:\Users\user\project\node_modules\progressbar.js\src\path.js:94)
at Circle.animate (C:\Users\user\project\node_modules\progressbar.js\src\shape.js:111)
at CircleBar.updateProgress (C:\Users\user\project\src\build_js\renderer\components\progress.js:9)
```

When I try to animate / set a bar :
```
let bar = new CircleBar("#progressbar");
bar.updateProgress(value);
```

Start with the reported call in src/build_js/renderer/components/progress.js and trace it through progressbar.js/src/path.js, then compare the stack with the shifty failure in getFormatChunksFrom. Reproduce the CircleBar updateProgress(value) example; done means animating or setting the bar no longer throws the reported TypeError.

Written by the indexing model from the issue text.