Player, playerObj, or closestCube.CubeMovement doesn’t exist.
Null Reference exceptions are always solvable exactly the same way, you don’t need to make threads for them. Simply do your get component calls and do a Debug.Log() of the variable before trying to use it. You will find the null reference easily.
The strange thing is that I only ask the arrivalAtTime of the closestCube, but it gives me all the cubes. Debug.Log(player.closestCube) just gives me only the closestCube.