Throw IllegalArgumentException when removing null child from figure

The Figure#remove() method throws a NullPointerException when trying to
remove a "null" child, due to a call to figure.getParent(). To remain
consistent with the case where the child belongs to a different parent,
this should also throw an IllegalArgumentException.

org.eclipse.draw2d/src/org/eclipse/draw2d/Figure.java

@@ -1370,7 +1370,7 @@ protected void primTranslate(int dx, int dy) {
 	 */
 	@Override
 	public void remove(IFigure figure) {
-		if ((figure.getParent() != this)) {
+		if (figure == null || figure.getParent() != this) {
 			throw new IllegalArgumentException("Figure is not a child"); //$NON-NLS-1$
 		}
 		if (getFlag(FLAG_REALIZED)) {