Bug 153523 OK button in parameter dialog is enabled without default value in combo-box
Description: OK button in parameter dialog is enabled without default value in combo-box Steps to reproduce: 1. New a report parameter, combo-box. 2. Click New button, input abc as value, click OK 3. Set abc as default value and close Edit Parameter dialog. 4. New a data source and dataset 5. Edit the parameter, remove abc value and import values from dataset Expected result: OK button is disabled for there is no default value. Actual result: OK button is enabled.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/CascadingParametersDialog.java
{
			if ( selectedParameter != null )
			{
				type = selectedParameter.getDataType( );
			}
			else
			{
				type = DesignChoiceConstants.PARAM_TYPE_STRING;
			}
		}
		else
		{
			IChoice choice = dataType.findChoiceByDisplayName( dataTypeChooser.getText( ) );
			type = choice.getName( );
		}
		return type;
	}
	/**
	 * Gets the internal name of the control type from the display name
	 */
	private String getSelectedDisplayType( )
	{
		String displayText = displayTypeChooser.getText( );
		if ( displayText.length( ) == 0 )
		{
			return null;
		}
		if ( DISPLAY_NAME_CONTROL_COMBO.equals( displayText ) )
		{
			return PARAM_CONTROL_COMBO;