Assembly: fixed defect in the migrationScript4 function

The old CAD model created by the development version of FreeCAD
(in which the Assembly workbench was used) contained joints that,
for some reason, had an empty Joint Connector 1 reference (Reference1).

This was causing an exception and a crash of the Python function
called migrationScript4. The FreeCAD Report view contained:

23:28:29  pyException: Traceback (most recent call last):
  File "/FreeCAD/Mod/Assembly/JointObject.py", line 175, in onDocumentRestored
    self.createProperties(joint)
  File "/FreeCAD/Mod/Assembly/JointObject.py", line 181, in createProperties
    self.migrationScript4(joint)
  File "/FreeCAD/Mod/Assembly/JointObject.py", line 514, in migrationScript4
    if hasattr(joint, "Reference1") and joint.Reference1[0] is not None:
                                        ~~~~~~~~~~~~~~~~^^^
<class 'TypeError'>: 'NoneType' object is not subscriptable

This patch attempts to avoid such exceptions by validating the type of
the property variable before accessing it.

src/Mod/Assembly/JointObject.py

@@ -28,6 +28,7 @@ import Part
 
 from PySide import QtCore
 from PySide.QtCore import QT_TRANSLATE_NOOP
+from collections.abc import Sequence
 
 if App.GuiUp:
     import FreeCADGui as Gui
@@ -511,7 +512,11 @@ class Joint:
             joint.Offset2 = App.Placement(current_offset, App.Rotation(current_rotation, 0, 0))
 
     def migrationScript4(self, joint):
-        if hasattr(joint, "Reference1") and joint.Reference1[0] is not None:
+        if (
+            hasattr(joint, "Reference1")
+            and isinstance(joint.Reference1, Sequence)
+            and joint.Reference1[0] is not None
+        ):
             doc_name = joint.Reference1[0].Document.Name
             sub1 = joint.Reference1[1][0]
             sub1 = UtilsAssembly.fixBodyExtraFeatureInSub(doc_name, sub1)
@@ -521,7 +526,11 @@ class Joint:
             if sub1 != joint.Reference1[1][0] or sub2 != joint.Reference1[1][1]:
                 joint.Reference1 = (joint.Reference1[0], [sub1, sub2])
 
-        if hasattr(joint, "Reference2") and joint.Reference2[0] is not None:
+        if (
+            hasattr(joint, "Reference2")
+            and isinstance(joint.Reference2, Sequence)
+            and joint.Reference2[0] is not None
+        ):
             doc_name = joint.Reference2[0].Document.Name
             sub1 = joint.Reference2[1][0]
             sub1 = UtilsAssembly.fixBodyExtraFeatureInSub(doc_name, sub1)