FLStudio Browser, locate the entry for the plugin you're having issues with in 'Plugin database > Generators' and 'Plugin database > Effects'.
If the plugin entry is present right-click it and select 'Delete file...'.
Once you've confirmed ...
fl into harmor or phaseplant even serum or whatever else. its simple but is definitely a sick addition. I use FL everyday, its my main daw, I've been using it for 4 years, since FL10 from what i can remember ...