How to fix missing JFrame library/class?

Question

As seen in screenshot 1, NetBeans (v8.2) does not recognise JFrame when extended (this is how NetBeans generated the file, which worked fine on another computer). As seen in screenshot 2, JFrame does not appear in the package list.

I have tried:

1. Creating a new project
2. Changing the line to extends JFrame
3. Restarting IDE
4. Reinstalling IDE
5. Googling a lot (no one else seems to have this issue)
6. Listed the bug at https://issues.apache.org/jira/browse/NETBEANS-905

Any ideas?

Screenshot 1:

Screenshot 2:

Answer 1

Followed a suggestion from Laszlo Kishalmi at issues.apache.org:

1. Went to Help -> About in NetBeans.
2. Copied filepath for cache folder in "About" window, closed NetBeans.
3. Browsed the folder, deleted it.
4. Re-opened NetBeans and the issue was solved.

Sorted!