Exporter throws exception in _detectTextures if image is None

[tngreene]

If a person tries to export with Autodetect Texture on when texture has no image selected, slot.texture.image.filepath will throw AttributeError on a line like xplane_material.py, line 180

                    self.texture = slot.texture.image.filepath

A potential fix

            if slot and slot.use and slot.texture.type == 'IMAGE' and slot.texture.image is not None: