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/ParameterDialog.java
if ( valueTable.getSelection( ).isEmpty( ) )
		{
			isEnable = false;
		}
		else
		{
			selectedChoice = (SelectionChoice) ( (IStructuredSelection) valueTable.getSelection( ) ).getFirstElement( );
			
			
			
			
		}
		boolean isDefault = isEnable && isDefaultChoice( selectedChoice );
		if ( isDefault )
		{
			changeDefault.setText( BUTTON_LABEL_REMOVE_DEFAULT );
		}
		else
		{
			changeDefault.setText( BUTTON_LABEL_SET_DEFAULT );
		}
		changeDefault.setSelection( isDefault );
		changeDefault.setEnabled( isEnable );
	}
	private void updateButtons( )
	{
		boolean canFinish = !StringUtil.isBlank( nameEditor.getText( ) );
		if ( canFinish )
		{
			if ( errorMessageLine != null && !errorMessageLine.isDisposed( ) )