shadow support for differently sized patterns

qtgui/pattern_grid.py

@@ -425,8 +425,15 @@ class PatternGrid(QtWidgets.QGraphicsScene):
     def createShadow(self, exportDict):
         """Receives steps from another track and display them as shadoy steps in the current one
         as a reference. Creating a new shadow does not delete the old one.
+
+        It is possible that the source pattern is longer (multiplicator) than our own. We ignore
+        all shadows beyond our own length.
+
+        Same is true if the source has more notes (pitches) than ours. We do naive x/y mapping.
+        Making sense of that is up to the user :)
         """
         for x, y in ((s["index"], s["pitch"]) for s in exportDict["pattern"]):
+            if (x,y) in self._steps:
                 self._steps[(x,y)].shadow = True   # (x,y):Step()
                 self._steps[(x,y)].setApperance()