This link suggests the problem might be related to Java version:
And this link suggests that maybe your Netbeans "userDir" has become corrupt:
Unfortunately, the link it points to from there doesn't exist anymore:
<= Dead link
But this looks like a "Bingo!":
At times your IDEs userdir gets corrupted and this results in the IDE behaving weirdly like:
1) In the New Project dialog, sometimes only a few of the categories are listed.
2) Your Project does not open properly, you get errors like "Project Folder Already exist" etc
3) You get an exception window on IDE startup etc.
To fix such issues, run the ide with a fresh userdir.
To do this:
a) Delete your userdir entirely,
b) restart NetBeans, and allow it to generate a new userdir from scratch.
c) This "reset" should repair the problem.
Also, instead of deleting the userdir, you can go to etc/netbeans.conf and change the value of the userdir key to something else. Then the something else will be the new userdir when next you start the IDE.
'Hope that helps .. PSM