There are several issue reporting NullPointerException and in logs even several related to hints (In context of Tools > Options > Editor > Hints).
So my suggestions are:
1) to be deactivated all hints and retested with and with and without nb-javac as seems that many issue like this is connected nb-javac, but It could be even possible that hints are not revised at all from start and even it is possible that could be ticked to run by import from prior version of Netbeans, but it could be unsupported in new version.
2) turn off of nb-javac does not fix issues like these, because it seems that that states is ignored and nb-javac is still somehow in use ... so only option is to uninstalled completely (so to see in notification request for installation) - so it has to be fixed that if it is inactivated should not be used
3) to be added even new feature to reset all Hints to default
4) seems that even some of such failure (even reported as race condition) that leads to cases that it is not finished indexing so particular classes are not index so even not visible by dialog to of types and in java editor is represented as errors and there is not way to removed that reds, because there are no way to tell to Apache Netbeans that such class is existing and it is correct.
5) "Background scanning of projects..." is even running for minutes with nb-javac installed
Even if it is not done in minor version I would like to see it in major version fixed.
To me it seems that nb-javac brings more issue than save