ERROR:
Blueprint could not be loaded because it derives from an invalid class. Check to make sure the parent class for this blueprint hasn’t been removed! Do you want to continue (it can crash the editor)?
VERSION: Still happens in 5.2.1. (even when using plugins like ImGui)
SOLUTION: Deleted “Binaries” folder (as mentioned by @AleAB94 and @sandolkakos) and its working so far. I don’t know if this is releevant or not, but after deleting “Binaries”, I right click on the .uproject file and then “Generate Visual Studio Project Files” (as an added step), then double click on the .uproject file, and it started working.