Don't try to read values from null cameras and lights in GLTF

modules/gltf/extensions/gltf_light.cpp

@@ -109,6 +109,7 @@ void GLTFLight::set_outer_cone_angle(float p_outer_cone_angle) {
 Ref<GLTFLight> GLTFLight::from_node(const Light3D *p_light) {
 	Ref<GLTFLight> l;
 	l.instantiate();
+	ERR_FAIL_COND_V_MSG(!p_light, l, "Tried to create a GLTFLight from a Light3D node, but the given node was null.");
 	l->color = p_light->get_color();
 	if (cast_to<DirectionalLight3D>(p_light)) {
 		l->light_type = "directional";

modules/gltf/structures/gltf_camera.cpp

@@ -58,6 +58,7 @@ void GLTFCamera::_bind_methods() {
 Ref<GLTFCamera> GLTFCamera::from_node(const Camera3D *p_camera) {
 	Ref<GLTFCamera> c;
 	c.instantiate();
+	ERR_FAIL_COND_V_MSG(!p_camera, c, "Tried to create a GLTFCamera from a Camera3D node, but the given node was null.");
 	c->set_perspective(p_camera->get_projection() == Camera3D::ProjectionType::PROJECTION_PERSPECTIVE);
 	// GLTF spec (yfov) is in radians, Godot's camera (fov) is in degrees.
 	c->set_fov(Math::deg_to_rad(p_camera->get_fov()));