fix crash with MultiMeasureRest export for cursor and lilypond when removing the metrical context beforehand

engine/items.py

@@ -2073,6 +2073,12 @@ class MultiMeasureRest(Item):
     def _lilypond(self, carryLilypondRanges):
         """Called by block.lilypond(carryLilypondRanges), returns a string.
         See Item.lilypond for the general docstring."""
+
+        if not self._cachedOneMeasureInTicks:
+            #it is possible to end up with MM-Rests in a track without any metrical instruction. Linked blocks, deleting the metrical after placing MMRests etc.
+            #This would crash the next lookup because _cachedMetricalInstruction is None
+            return ""
+
         lyduration = "{}/{}".format(self._cachedMetricalInstruction.nominator, duration.baseDurationToTraditionalNumber[self._cachedMetricalInstruction.denominator])
 
         if self.lilypondParameters["visible"]: