Fix T48397: Can not bake tweaked NLA

We need to leave tweak mode before trying to modifiy the action as doing
so will leave Blender in a semi-corrupted state.

Reviewers: #animation

Reviewed by: aligorith

Maniphest Tasks: T48397

Differential Revision: https://developer.blender.org/D2119

release/scripts/modules/bpy_extras/anim_utils.py

@@ -155,6 +155,11 @@ def bake_action(frame_start,
     atd = obj.animation_data_create()
     if action is None:
         action = bpy.data.actions.new("Action")
+
+    # Leave tweak mode before trying to modify the action (T48397)
+    if atd.use_tweak_mode:
+        atd.use_tweak_mode = False
+
     atd.action = action
 
     # -------------------------------------------------------------------------