Null is computer term for nothing, void, empty, or Zero. In this case, it refers to the Latitude/Longitude coordinates 0.0,0.0. Which happens to be in the ocean near Ghana.
We've put a marker here, so you can find out about the point, as it happens every now and then, when your tracking device is unable to see enough Satellites to get a location fix, it records 0,0 for it's location.
Please allow several minutes (outdoors) for accurate Satellite location of your tracker to automatically rectify this.
More info: Null Island wiki page