RuntimeError When Adding Part

[nickfong]

The following error is encountered when a part is added, the addition of the part is undone, and another part is added:

Traceback (most recent call last):
  File "./cadnano/views/gridview/gridrootitem.py", line 92, in partAddedSlot
    self.select_tool.setPartItem(na_part_item)
  File "./cadnano/views/gridview/tools/selectgridtool.py", line 147, in setPartItem
    self.group.resetTransform()
RuntimeError: wrapped C/C++ object of type GridSelectionGroup has been deleted
Abort trap: 6

To reproduce:

1. Click HoneyComb
2. Undo
3. Click HoneyComb or Square
4. cadnano2.5 crashes with the aforementioned error

On master with HEAD at 0a160f3