Add null check for NavigationMesh.create_from_mesh()

(cherry picked from commit cf6bfea93ff2a1e2b166de07bc8a0625230f3cad)

scene/3d/navigation_mesh.cpp

@@ -33,6 +33,7 @@
 #include "navigation.h"
 
 void NavigationMesh::create_from_mesh(const Ref<Mesh> &p_mesh) {
+	ERR_FAIL_COND(p_mesh.is_null());
 
 	vertices = PoolVector<Vector3>();
 	clear_polygons();