problem with ConfluentMonoidPresentationForGroup or IsomorphismFpGroupByChiefSeries

[ThomasBreuer]

The following happens in GAP 4.13.1 and in the current master branch.

gap> G:= Omega( -1, 6, 3 );;
gap> ConfluentMonoidPresentationForGroup( G );
Error, List Element: <list>[73] must have an assigned value in
  img := opr( img, S.transimages[bimg[1]] )
 ; at /.../lib/ghomperm.gi:132 called from 
ImageSiftedBaseImage( S, OnTuples( BaseStabChain( S ), elm ), S.idimage, 
 OnRight ) at /.../lib/ghomperm.gi:1076 called from
ImagesRepresentative( RestrictedInverseGeneralMapping( hom ), elm 
 ) at /.../lib/ghom.gi:273 called from
PreImagesRepresentative( hom, j 
 ) at /.../lib/gpfpiso.gi:400 called from
func( C[i] ) at /.../lib/coll.gi:663 called from
List( fgens, function ( j )
      return Image( hom, PreImagesRepresentative( hom, j ) ^ i );
  end ) at /.../lib/gpfpiso.gi:400 called from
...  at *stdin*:2
type 'quit;' to quit to outer loop

In GAP 4.12.2, I get another error.

gap> G:= Omega( -1, 6, 3 );;
gap> ConfluentMonoidPresentationForGroup( G );
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `IsomorphismFpGroupByChiefSeries'
[...]

Since GAP 4.13, apparently IsomorphismFpGroupByChiefSeries has a method for the given group, but we run into the error inside this method.
If we switch to an isomorphic permutation group before calling ConfluentMonoidPresentationForGroup then ConfluentMonoidPresentationForGroup( G ) succeeds in GAP 4.13.1 or newer, and I was not patient enough to wait for the answer in GAP 4.12.2.

(The problem has been originally observed by @fieker.)