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
{
			doPreview( formatCategroy != DesignChoiceConstants.STRING_FORMAT_TYPE_CUSTOM ? formatCategroy
					: formatPattern );
		}
	}
	private void doPreview( String pattern )
	{
		String type = getSelectedDataType( );
		String formatStr = ""; 
		if ( DesignChoiceConstants.PARAM_TYPE_STRING.equals( type ) )
		{
			formatStr = new StringFormatter( pattern, ULocale.getDefault( ) ).format( DEFAULT_PREVIEW_STRING );
		}
		else if ( DesignChoiceConstants.PARAM_TYPE_DATETIME.equals( type ) )
		{
			formatStr = new DateFormatter( pattern ).format( new Date( ) );
		}
		else if ( DesignChoiceConstants.PARAM_TYPE_DECIMAL.equals( type )
				|| DesignChoiceConstants.PARAM_TYPE_FLOAT.equals( type ) )
		{
			formatStr = new NumberFormatter( pattern ).format( DEFAULT_PREVIEW_NUMBER );
		}
		else if ( DesignChoiceConstants.PARAM_TYPE_INTEGER.equals( type ) )
		{
			formatStr = new NumberFormatter( pattern ).format( DEFAULT_PREVIEW_INTEGER_NUMBER );
		}
		previewLable.setText( formatStr );
	}
	private String getColumnName( ScalarParameterHandle handle, String column )
	{