title: Blender IndexError: bpy_prop_collection
body: <p>I try write a game with GoranM/bdx plugin. When i create plate with texture and  try export to code I get fatal error.</p>
  
  <blockquote>
    <p>Traceback (most recent call last):
      File "C:\Users\Myuser\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons\bdx\ops\exprun.py", line 225, in execute
        export(self, context, bpy.context.scene.bdx.multi_blend_export, bpy.context.scene.bdx.diff_export)
      File "C:\Users\Myuser\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons\bdx\ops\exprun.py", line 123, in export
        bpy.ops.export_scene.bdx(filepath=file_path, scene_name=scene.name, exprun=True)
      File "C:\Program Files\Blender Foundation\Blender\2.79\scripts\modules\bpy\ops.py", line 189, in <strong>call</strong>
        ret = op_call(self.idname_py(), None, kw)
    RuntimeError: Error: Traceback (most recent call last):
      File "C:\Users\Myuser\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons\bdx\exporter.py", line 903, in execute
        return export(context, self.filepath, self.scene_name, self.exprun, self.apply_modifier)
      File "C:\Users\Myuser\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons\bdx\exporter.py", line 829, in export
        "models": srl_models(objects, apply_modifier),
      File "C:\Users\Myuser\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons\bdx\exporter.py", line 117, in srl_models
        verts = vertices(mesh)
      File "C:\Users\Myuser\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons\bdx\exporter.py", line 53, in vertices
        vert_uv = list(uv_layer[li].uv)
    IndexError: bpy_prop_collection[index]: index 0 out of range, size 0</p>
    
    <p>location: C:\Program Files\Blender Foundation\Blender\2.79\scripts\modules\bpy\ops.py:189</p>
    
    <p>location: :-1</p>
  </blockquote>
  
  <p>Maybe someone had same problem and you know how to fix it?</p>