glTF: Removing one model breaks an other model

Reproduce:
- Insert two models
- Play first model
- Delete second model
Result: crash (windows) or first model disappear from the
opengl window (linux).

Change-Id: I940960754f42a2e4faf5b9f37c0d4a3c155e4650
glTF: crash after starting a model without an OpenGL window

Change-Id: I726f4d62ab062fbd41376183f60c26d8e6cfe33e