Updated ENM to avoid situation where network is applied to single-stranded DNA...

Updated ENM to avoid situation where network is applied to single-stranded DNA at end of double-stranded helix

mrdna/segmentmodel.py

@@ -3510,7 +3510,12 @@ class SegmentModel(ArbdModel):
                         nt2 = nt1.get_intrahelical_above()
                         if nt2 is not None:
                             other.append((nt2,'stack'))
-                            nt2 = nt2.basepair
+                            try:
+                                nt2 = nt2.basepair
+                            except:
+                                assert( isinstance(nt2.parent.segment, SingleStrandedSegment) )
+                                nt2 = None
+
                             if nt2 is not None and strand_piece.is_fwd:
                                 other.append((nt2,'cross'))