Check multimesh before uniform set creation

servers/rendering/renderer_rd/storage_rd/mesh_storage.h

@@ -614,6 +614,9 @@ public:
 
 	_FORCE_INLINE_ RID multimesh_get_3d_uniform_set(RID p_multimesh, RID p_shader, uint32_t p_set) const {
 		MultiMesh *multimesh = multimesh_owner.get_or_null(p_multimesh);
+		if (multimesh == nullptr) {
+			return RID();
+		}
 		if (!multimesh->uniform_set_3d.is_valid()) {
 			if (!multimesh->buffer.is_valid()) {
 				return RID();
@@ -632,6 +635,9 @@ public:
 
 	_FORCE_INLINE_ RID multimesh_get_2d_uniform_set(RID p_multimesh, RID p_shader, uint32_t p_set) const {
 		MultiMesh *multimesh = multimesh_owner.get_or_null(p_multimesh);
+		if (multimesh == nullptr) {
+			return RID();
+		}
 		if (!multimesh->uniform_set_2d.is_valid()) {
 			if (!multimesh->buffer.is_valid()) {
 				return RID();