Quick fix: handle exceptions in updateModelFromPOST().
Method GeneralCallbackDefault::groupCallback expect an model object, not an array.
Fix custom parent field in mode 4.

If the parent reference field is not named pid,
the name must read from getParentChildCondition().
Merge pull request #81 from tristanlins/contao3

Merging Quick fix: handle exceptions in updateModelFromPOST().

This need to be refined though.