Unity Crashes after Deleting Imported Packages

This might be specific to NGUI, but I doubt it. I want to remove NGUI from my project. Just deleting it from the folder doesn’t work because Unity will not open after this. Is there a specific “remove imported library” button that I’m not seeing?

There didn’t seem to be anything referencing or using the package. I deleted the package from within Unity (rather than the folder in Explorer) and that worked. Not sure what was going on