Bug 182673 No default value error message prompts incorrectly
Steps to reproduce: 1. New a report parameter; 2. Choose control type as Combo; 3. Choose parameter as dynamic type; Expected result: As the isRequired is false by default, parameter's default value can be null value. There should be no error message about no default value prompt. Actual result; Error messages prompts
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/CascadingParametersDialog.java
{
				defaultValueChooser.setText( defaultValue );
			}
		}
		helpTextEditor.setText( UIUtil.convertToGUIString( selectedParameter.getHelpText( ) ) );
		if ( selectedParameter.getPropertyHandle( ScalarParameterHandle.LIST_LIMIT_PROP )
				.isSet( ) )
		{
			listLimit.setText( String.valueOf( selectedParameter.getListlimit( ) ) );
		}
		else
		{
			listLimit.setText( "" ); 
		}
		
		isRequired.setSelection( selectedParameter.isRequired( ) );
		changeDataType( selectedParameter.getDataType( ) );
		loading = false;
	}
	private void clearParamProperties( )
	{
		paramNameEditor.setText( "" ); 
		promptText.setText( "" ); 
		dataTypeChooser.select( -1 );
		displayTypeChooser.select( -1 );
		defaultValueChooser.setText( "" ); 
		helpTextEditor.setText( "" ); 
		formatField.setText( "" ); 
		listLimit.setText( "" ); 
		previewLable.setText( "" );