On the last page of the restore dialog you see the "Source Options" with 2 lines of information:
"Restore from original pool" showing an empty list (this causes the Java error).
"Restore from alternate pool" showing a filled list of pools.
Cause: different backup server name settings "Default Address" and "Hostname" in NodesManagement
Workaround 1: use "Restore from alternate pool" in Restore Options and choose the original mediapool from the list.
Workaround 2: in NodesManagement change backup server "Hostname" to the value shown for "Default address". That is GUI->NodesManagement->"contextmenu on Backupserver"->EditNode->ConfigureAddress->ManageAddresses.
Workaround 1 is valid only for the chosen restore.
Workaround 2 is permanent.