As suggested in the comments, I 'checked every form and component'. I already knew that it wasn't an event property that was causing the problem, since I was already familiar with all events.
But I did find two combo-boxes that I did not expect to see.
As discussed at Empty string in Delphi / Windows combo box causes access exception, there is a known problem with Delphi 7.0 combo-boxes that causes a access-violation error when you access an empty line: http://qc.embarcadero.com/wc/qcmain.aspx?d=2246.
For no obvious reason, this known problem was triggered when the parent of the form was set.
Possible solutions include removing the combobox, patching the library, applying the 7.1 service pack, or upgrading.
In this case, the problem was resolved by removing the empty lines from the default string list of the combo-box.