Found an issue with the gradient node when trying to copy paste values.

Reproduce steps:

1. Create a gradient node
2. Copy the gradient value
3. Paste the gradient value back to itself
4. Try to save the project
5. Crashes

This also happens when you try to copy/paste from one gradient node to another, that's how I found it. But the crash can be replicated by just one gradient node and copy/pasting the value inside it. Crashes when attempting to save the project (unable to serialize the node).

Also added to github issues list
https://github.com/Ttanasart-pt/Pixel-Composer/issues/92

Noted, thanks for reporting.