Do not dispose a system defined color platform resource

System Grey colour is being disposed by accident. This breaks other Eclipse UI plugins for obvious reasons.
This is a quick fix for the issue: do not dispose that specific colour.

I have noticed that you create and dispose colours manually. Feels like that has lead to this problem. See if perhaps you could use org.eclipse.jface.resource.ColorRegistry for caching colours. There is a similar class for Fonts as well.