This one’s a weird one. Sometimes Flash Builder 4.7 has a problem where weird errors come up in library code (Feathers, usually) that doesn’t have any actual errors in it.

The error raised is the following: com.google.common.collect.ComputationException: java.lang.NullPointerException

The (non-obvious) solution is to find a little button called “Mark Occurrences” in the toolbar and turn it to OFF.

Here’s a picture of what the button looks like when set to OFF:

And here’s what it looks like when set to ON:

Oh, and sometimes restarting Flash Builder doesn’t do it, so sometimes in order to fix it, I have to set mark occurrences to ON, close and restart FB, and then set it to OFF again to get it to work.

More info is available in this post in the Feathers forum here.