Actually BPMN2 specify a few rules that do not allow saving the diagrams (for further work on it) until it's completely validated against all bpmn2 rules.

Consider as example these situations:

• Creating a new BPMN2 diagram generates a new graph with an initial start event node added by default -> it cannot be saved (and implicitly validated) until an end event is added into the graph.
• When opening an existing BPMN2 diagram, if there's only a single end event present, it cannot be removed until a new end event is added into the graph as well, due to a valid bpmn2 must contain an end event. Same happens for start events.