Crash when editing object properties

This should be reproducible:

1. Connect to the database of your choice.

2. Navigate to the "Tables" node in the tree view and select it. The 
list of tables appears in the upper right pane.

3. Select a table in the upper right pane.

4. Click the "Display/Edit Properties" button in the toolbar. *BOOM*

This also happens with table columns, functions and trigger functions, 
but interestingly, not always with sequences.

i.