Warning array to string conversion in oik_box when $id null and $callback is a method

[bobbingwide]

While developing an improvement to oik-clone I coded the following calls to oik_box().

oik_box( null, null, "Slave", [ $this, 'oik_clone_slave_form' ] );

I got a warning message because of this code.

if ( $id == null ) {
			$id = $callback;
		}  

The code should allow for the $callback parameter being an array representing a method in a class.

Workaround

Pass a unique value for the $id parameter, rather than null.