Crash in WriteLocker

Steps to reproduce:

• Create an empty project
• Select root item (Rectangle)
• Change fill color from #c2c2c2 to #c2c2 (invalid)
• Select other item (e.g. Button)
• Popup is shown "Invalid property name "color". (M16)
• Click OK
• Select root item again
• Change fill color from #c2c2c2 to #c2c2 (invalid)
• Select other item (e.g. Button)
• CRASH