It's currently possible to create an invalid PlanarCross.
Create one normally, modify one control-point set the third control-point and do not move the mouse.
-> The fourth control-point is positioned falsely the position it was located at before resetting.
The porblem behind this is the a bug in the new control-point datatypes. A new control-point is appended to the list. However, this point is at control-point list position 5 instead of 4 because of the previously set point
-> simply resize the list if the maximum number of control-points would be overrun...