It's possible to destroy the underlying GraphicsContextGL before WebXROpaqueFramebuffer's destructor runs. For example, when creating too WebGL contexts, the context associated with a WebXROpaqueFramebuffer will be lost. The GCGLOwned objects exist to catch when the user forgets to clean up. In this case, where the GraphicsContextGL is NULL at destruction time, there's nothing to do but reset the GCGLOwned objects, preventing their destructors from asserting in debug builds.