fix NullPointerException in DropController.handleDrop()

The problem was, that DropDeserialiser drops unknown messages without any info. It uses the default system ClassLoader which wasn't aware of the jar which contains the module. so loading a ModelObject resulted in an Exception which was silently ignored and returned null.

The DropController does not expect null-values, so it added the null-value to the list of received messages. afterwards when handleDrop was called on the null-value, it tried to get the module object from null and therefore crashed.