AttributeError: 'NoneType' object has no attribute 'key_blocks'
**Describe the bug**

A Python error shows on export 

**To Reproduce**

1. Download the blend file below
2. Select the `rig` and the `body` objects
3. Export as gltf. Options used (different from defaults): 
    1. Include -> Selected objects
    2. Geometry -> Apply modifiers
4. See the traceback: https://pastebin.com/8cWeqAjz

**Expected behavior**

No error on export

**.blend file/ .gltf**

[mannequiny-0.2.0.zip](https://github.com/KhronosGroup/glTF-Blender-IO/files/3882867/mannequiny-0.2.0.zip)

**Version**
 - OS: linux
 - Blender Version 2.81

**Additional context**

I was unable to pinpoint the step that triggered the bug. It happened several times today with several versions of this blend file, and the only solution I've found was to revert to a previously saved version.
diff --git a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py
index 3125828f..6f10500d 100644
--- a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py
+++ b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py
@@ -94,6 +94,11 @@ def __get_channel_group_sorted(channels: typing.Tuple[bpy.types.FCurve], blender
         first_channel = channels[0]
         object_path = get_target_object_path(first_channel.data_path)
         if object_path:
+            if not blender_object.data.shape_keys:
+                # Something is wrong. Maybe the user assigned an armature action
+                # to a mesh object. Returning without sorting
+                return channels
+
             # This is shapekeys, we need to sort channels
             shapekeys_idx = {}
             cpt_sk = 0
['addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_animation_channels.py']