PowerPivot fieldlist is grey and not selectable

Sometimes my PowerPivot field list is grey and can't be selected. It can happen when you are busy in an existing PowerPivot sheet. It looks like a bug because a restart of Excel solved my situation.

Solution:

• Save your Power Pivot Sheet
• Close all other Excel sheets
• Close Excel
• Check in task manager if Excel.exe is not running
• If Excel.exe is still running, kill this task
• Start Excel
• Open your Power Pivot sheet
• Click on a graph and you should be able to select the Field list.

Unfortunatly, is is not clear when and how this happens. I'm using Power Pivot version 11.0.3000.0