solution what to do if you are using customized ui and you cannot open your featplanner.

I am copying what Sharum said in old forums, might be useful in future:

"If you're interested in updating your customized UI here's what you'll need to change:

This is the file you'll be editing:
Data/Gui/Customized/Views/MainGUI/MainSPlitView.xml

Look for:
SidepanelTab name="SPTabFeats".

Change the following lines:
left_module = "SPFeatsLeft"
right_module = "SPFeatsRight"

So that they read:
left_module = "FeatPlanner"
right_module = "FeatPlanner"

"