Crash due to changing a proto parameter while regenerating it

[omichel]

trafficstars

It seems that the #3214 crash might be related to this: https://github.com/cyberbotics/webots/blob/d2d792c13f52076200a3304d1c00a0b60be3ee07/src/webots/vrml/WbProtoModel.cpp#L269-L289 that determines if a change of a parameter should trigger a regeneration is if something like fields. is found. So if the Robocup referee sends commands through this field it might trigger other re-generations. It should also be noted that the Robocup referee will change this customData value (that will cause a re-generation) and reset the robot at the same time, which could be a bad combination leading to the crash of Webots.