While using the connect button in the debug interface GUI there is an option to load the bit file using the "Load Bitfile" checkbox. If a bit file has already been loaded then this box needs to be un-checked.
In the debug interface released with DPD v7.1 and DPD v7.1rev1 it is seen that after unchecking Load Bitfile, the debug interface is unable to connect and crashes with an error.
This happens because the code references a variable not yet defined in this specific control flow.
This known issue has been fixed in a new rev2 release of the debug interface.
The DPD debug interface release 7.1 rev-2 patch can be downloaded from the DPD lounge: