Fix make single user crash

How to reproduce the crash:
* Factory startup
* 'u'key (make single user)

It comes with a simple unittest to reproduce the original issue.

tests/python/view_layer/test_make_single_user.py

@@ -0,0 +1,54 @@
+# ############################################################
+# Importing - Same For All Render Layer Tests
+# ############################################################
+
+import unittest
+import os
+import sys
+
+from view_layer_common import *
+
+
+# ############################################################
+# Testing
+# ############################################################
+
+class UnitTesting(ViewLayerTesting):
+    def test_make_single_user(self):
+        """
+        Really basic test, just to check for crashes on basic files.
+        """
+        import bpy
+        scene = bpy.context.scene
+        master_collection = scene.master_collection
+        view_layer = bpy.context.view_layer
+        ob = bpy.context.object
+
+        # clean up the scene a bit
+        for o in (o for o in view_layer.objects if o != ob):
+            view_layer.collections[0].collection.objects.unlink(o)
+
+        for v in (v for v in scene.view_layers if v != view_layer):
+            scene.view_layers.remove(v)
+
+        while master_collection.collections:
+            master_collection.collections.remove(
+                    master_collection.collections[0])
+
+        view_layer.collections.link(master_collection)
+        ob.select_set('SELECT')
+
+        # update depsgraph
+        scene.update()
+
+        # test itself
+        bpy.ops.object.make_single_user(object=True)
+
+
+# ############################################################
+# Main - Same For All Render Layer Tests
+# ############################################################
+
+if __name__ == '__main__':
+    UnitTesting._extra_arguments = setup_extra_arguments(__file__)
+    unittest.main()