Bug 202254 Exception is thrown when category values are null and type of X axis is set to DateTime
chart/org.eclipse.birt.chart.ui.extension/src/org/eclipse/birt/chart/ui/swt/wizard/format/axis/AbstractAxisSubtask.java
private Combo cmbTypes;
	private Combo cmbOrigin;
	private Button btnFormatSpecifier;
	private Label lblValue;
	private TextEditorComposite txtValue;
	private Button btnLabelVisible;
	private FontDefinitionComposite fdcFont;
	private Button cbStaggered;
	private Button btnAxisTitle;
	private Button btnAxisLabel;
	AbstractAxisSubtask( )
	{
		super( );
	}
	abstract protected Axis getAxisForProcessing( );
	/**
	 * Returns the axis angle type
	 * 
	 * @return <code>AngleType.X</code>, <code>AngleType.Y</code> or
	 *         <code>AngleType.Z</code>
	 */
	abstract protected int getAxisAngleType( );
	public void createControl( Composite parent )
	{
		cmpContent = new Composite( parent, SWT.NONE );
		{
			GridLayout glContent = new GridLayout( 2, false );
			cmpContent.setLayout( glContent );
			GridData gd = new GridData( GridData.FILL_BOTH );
			cmpContent.setLayoutData( gd );