Rebuild IntelliJ project indexes

Restarting IntelliJ fixed this in my case (Ultimate 2019.1) -- on restarting, it rebuild the indexes by itself.

An interesting side effect, apart from all imports and non-local references coming up as errors, was the error reported on all string literals:

Incompatible types. 
Required: java.lang.String 
Found: java.lang.String

File -> Invalidate caches...

(Force rebuild of all caches and indices on next startup)

Restart IntelliJ IDEA.

Caution: This might Remove local history.

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