Plot Crashes after deleting Slots or Objects [6020/6021]
Bug Number: 6020 and 6021
Release notes (y/n): Yes
For Release Nums: 7.1.4 and 7.2

6020: Crash opening Plot dialog when slots in plots have been deleted
6021: Crash when scaling a Plot that includes slots from a deleted object

Adding a simple NULL pointer check for a slot pointer to this method fixed 
all three (3) crash scenarios described in these two bug reports.

  unit_type SlotCurve::getSlotUnitType(Slot *slotPtr, int colNum).

I examined the stack (in the debugger) when a NULL slot pointer was 
encountered (at the point of the added NULL pointer check), and didn't 
see a higher level in which a similar check was was warranted.  Also, 
I examined similar methods in the SlotCurve classes, and didn't see any 
others which needed the addition of this sort of check.

The fact that a NULL slot pointer -- rather than an invalid pointer -- was 
produced in all three crash scenarios is a good thing.  The single change 
made for this fix is sufficient.

[Mitch -- great job on those bug reports!  Very thorough].

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