cannot save or load in Sketch 53
I was previously able to save a palette from this same file, and was previously able to load that palette in other files. Now, nothing happens in the UI when I try to save a palette or try to load a previously created palette.

Here's the output of my `crash.log` from the Plugins folder: 

```
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<array>
	<string>instant HandlerGotFocus {
    name = Normal;
}</string>
	<string>begin SelectionChanged {
    document = "&lt;MSDocument: 0x7fe746a80350&gt;";
    newSelection =     (
    );
    oldSelection =     (
    );
}</string>
	<string>finish SelectionChanged {
    document = "&lt;MSDocument: 0x7fe746a80350&gt;";
    newSelection =     (
    );
    oldSelection =     (
    );
}</string>
	<string>begin RunPlugin {
    action = "&lt;MSRunPluginAction: 0x6000005c8810&gt;";
}</string>
	<string>begin RunPluginCommand {
    command = "com.andrewfiorillo.sketch.sketch-palettes/savePalette";
    document = "&lt;MSDocument: 0x7fe746a80350&gt;";
    selection =     (
    );
}</string>
	<string>begin Log {
    action = "&lt;MSLogAction: 0x600001712e00&gt;";
    command = "&lt;MSPluginCommand: 0x600006726700&gt;";
    level = error;
    payload =     (
        "TypeError: assets.colors is not a function. (In 'assets.colors()', 'assets.colors' is undefined)"
    );
    stringValue = "TypeError: assets.colors is not a function. (In 'assets.colors()', 'assets.colors' is undefined)\nline: 91\nsourceURL: /Users/chad/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Sketch Palettes.sketchplugin/Contents/Sketch/sketchPalettes.js\ncolumn: 34\nstack: setCheckboxStates@/Users/chad/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Sketch Palettes.sketchplugin/Contents/Sketch/sketchPalettes.js:91:34\nsavePalette@/Users/chad/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Sketch Palettes.sketchplugin/Contents/Sketch/sketchPalettes.js:105:19\n";
}</string>
	<string>finish Log {
    action = "&lt;MSLogAction: 0x600001712e00&gt;";
    command = "&lt;MSPluginCommand: 0x600006726700&gt;";
    level = error;
    payload =     (
        "TypeError: assets.colors is not a function. (In 'assets.colors()', 'assets.colors' is undefined)"
    );
    stringValue = "TypeError: assets.colors is not a function. (In 'assets.colors()', 'assets.colors' is undefined)\nline: 91\nsourceURL: /Users/chad/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Sketch Palettes.sketchplugin/Contents/Sketch/sketchPalettes.js\ncolumn: 34\nstack: setCheckboxStates@/Users/chad/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Sketch Palettes.sketchplugin/Contents/Sketch/sketchPalettes.js:91:34\nsavePalette@/Users/chad/Library/Application Support/com.bohemiancoding.sketch3/Plugins/Sketch Palettes.sketchplugin/Contents/Sketch/sketchPalettes.js:105:19\n";
}</string>
	<string>finish RunPluginCommand {
    command = "com.andrewfiorillo.sketch.sketch-palettes/savePalette";
    document = "&lt;MSDocument: 0x7fe746a80350&gt;";
    selection =     (
    );
}</string>
	<string>finish RunPlugin {
    action = "&lt;MSRunPluginAction: 0x6000005c8810&gt;";
}</string>
	<string>begin OpenPluginPreferences {
    action = "&lt;MSOpenPluginPreferencesAction: 0x6000005c84b0&gt;";
}</string>
	<string>finish OpenPluginPreferences {
    action = "&lt;MSOpenPluginPreferencesAction: 0x6000005c84b0&gt;";
}</string>
</array>
</plist>
```

seems the way color assets are handled in Sketch 53 is different?

Details: 
- macOS 10.14.2
- Sketch 53 (72520)