xamarin forms mobile development

Question

Hi

 

I have the login application and i have used one label to show the error in xaml page & i am displaying an error through label when username and password are null(empty ) and user click on login  button that label error is displayed but when user enter the data in username and password that label error is not going to disappeare..

so plz give me the suggestion to disappeare the label error when user enters the data..