A quick check of the relevant JMapViewer source shows that your call to getMapPosition() invokes a nearby overload with checkOutside set to true. The result is null if the Point corresponding to the coordinates is outside the visible map.
if (checkOutside && (p.x < 0 || p.y < 0 || p.x > getWidth() || p.y > getHeight())) {
return null;
}
Instead, use one of the implementations that lets you set checkOutside to false explicitly. For example,
Point da = map1().getMapPosition(48.9225, 16.875, false);
or
Coordinate coord = new Coordinate(48.9225, 16.875);
Point da = map1().getMapPosition(coord, false);