reset the _definingSolver to null when the Constraint solver changes
reset the _definingSolver to null when the Constraint solver changes

babelsberg/babelsberg-js,psieg/babelsberg-js,babelsberg/babelsberg-js,babelsberg/babelsberg-js,psieg/babelsberg-js,psieg/babelsberg-js,babelsberg/babelsberg-js,psieg/babelsberg-js