DebugController is responsible for destroying debug sessions and it can
not do so past its own destructor. Currently, depending on the relative
timing of KDevelop exit and the 5 second interval of the singleshot
timer in MIDebugSession::stopDebugger(), a DebugSession may be destroyed
safely in time or not destroyed at all and possibly cause a crash by
accessing the already destroyed DebugController or its children.

Let us kill debugger processes and thus finalize the debug sessions'
states in ~DebugController(). We delay the killing of debugger processes
for as long as possible to give them a chance to exit on their own.

Check ICore::documentController() for nullptr in
BreakpointModel::updateMarks() and
DebugController::clearExecutionPoint() to prevent crashes when these
slots are invoked by DebugSession's state transition signals from
inside ~DebugController(), which is called after ~DocumentController().

BUG: 425991
FIXED-IN: 5.6.1