Fix showing layer view if there is no material

The layer view would request the colour of the material. If there is no material, that would give an error.

Contributes to issue CURA-3497.

plugins/CuraEngineBackend/ProcessSlicedLayersJob.py

@@ -179,9 +179,10 @@ class ProcessSlicedLayersJob(Job):
             # Single extruder via global stack.
             material_color_map = numpy.zeros((1, 4), dtype=numpy.float32)
             material = global_container_stack.findContainer({"type": "material"})
-            color_code = material.getMetaDataEntry("color_code")
-            if color_code is None:  # not all stacks have a material color
-                color_code = "#e0e000"
+            color_code = "#e0e000"
+            if material:
+                if material.getMetaDataEntry("color_code") is not None:
+                    color_code = material.getMetaDataEntry("color_code")
             color = colorCodeToRGBA(color_code)
             material_color_map[0, :] = color