Right clicking on category item (Action,Condition,Control...) and choosing remove crashes

[esromneb]

Doing a right click on any of Action,Condition,Control,Decorator,SubTree and choosing remove causes an instant crash.

terminate called after throwing an instance of 'std::out_of_range'
  what():  map::at
[1]    31602 abort (core dumped)  ./Groot

This occurs with an empty tree.

I think I have a working PR to fix this I will submit in about 5 mins

[facontidavide]

thanks for fixing it

[esromneb]

You're welcome!