Thanks for your quick replies and fixes, but I still got errors when adding an external DEM. I went into the code and found that there are more bugs.
In createTargetProduct() this line still causes problems with an external DEM:
Because the dem.getDescriptor() does not exist for a external DEM...
When I comment this out and re-run the operator, a new error occurs in computeTile() at this line:
final double demNoDataValue = dem.getDescriptor().getNoDataValue();
This happens because of the same reason.
Maybe as a suggestion, I looked into the InterferogramOp and there you have to provide a the no-data value of the external dem yourself ("externalDemNoDataValue") and they use the filename as name for the DEM. Would this a solution to fix these bugs?