Animation export without target node
From #515 

It seems that the following blend file export a target without any node

[open-book-gltf-issue.zip](https://github.com/KhronosGroup/glTF-Blender-IO/files/3258214/open-book-gltf-issue.zip)

diff --git a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather.py b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather.py
index a723e447..ec3e33bf 100644
--- a/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather.py
+++ b/addons/io_scene_gltf2/blender/exp/gltf2_blender_gather.py
@@ -59,7 +59,10 @@ def __gather_scene(blender_scene, export_settings):
 def __gather_animations(blender_scene, export_settings):
     animations = []
     for blender_object in blender_scene.objects:
-        animations += gltf2_blender_gather_animations.gather_animations(blender_object, export_settings)
+        # First check if this object is exported or not. Do not export animation of not exported object
+        obj_node = gltf2_blender_gather_nodes.gather_node(blender_object, export_settings)
+        if obj_node is not None:
+            animations += gltf2_blender_gather_animations.gather_animations(blender_object, export_settings)
     return animations
 
 
['addons/io_scene_gltf2/blender/exp/gltf2_blender_gather.py']