Bug 166315 Combo box doesn't allow blank when set string type in parameter editor
Combo box doesn't allow blank when set string type in parameter editor Steps to reproduce: 1.New a parameter, display type is Combo Box 2.Select data type as String Expected result: "Allow blank values" is enable. Actual result: "Allow blank values" is gray and can't be checked.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
createLabel( moreOptionSection, null ); 
		Composite checkBoxArea = new Composite( moreOptionSection, SWT.NONE );
		checkBoxArea.setLayout( UIUtil.createGridLayoutWithoutMargin( 2, false ) );
		checkBoxArea.setLayoutData( new GridData( GridData.FILL_BOTH ) );
		allowNull = new Button( checkBoxArea, SWT.CHECK );
		allowNull.setText( CHECKBOX_ALLOW_NULL );
		addCheckBoxListener( allowNull, CHECKBOX_ALLOW_NULL );
		allowBlank = new Button( checkBoxArea, SWT.CHECK );
		allowBlank.setText( CHECKBOX_ALLOW_BLANK );
		addCheckBoxListener( allowBlank, CHECKBOX_ALLOW_BLANK );
		doNotEcho = new Button( checkBoxArea, SWT.CHECK );
		doNotEcho.setText( CHECKBOX_DO_NOT_ECHO );
		addCheckBoxListener( doNotEcho, CHECKBOX_DO_NOT_ECHO );
		isHidden = new Button( checkBoxArea, SWT.CHECK );
		isHidden.setText( CHECKBOX_HIDDEN );
		addCheckBoxListener( isHidden, CHECKBOX_HIDDEN );
		needSort = new Button( checkBoxArea, SWT.CHECK );
		needSort.setText( CHECKBOX_SORT );
		addCheckBoxListener( needSort, CHECKBOX_SORT );
	}
	/**
	 * Set the input of the dialog, which cannot be null
	 * 
	 * @param input
	 *            the input of the dialog, which cannot be null
	 */
	public void setInput( Object input )
	{
		Assert.isNotNull( input );