import { DrawerActions } from "react-navigation"; //Import it on top

....other code

signUserOut = () => {
      firebase.auth().signOut()
        .then( response => {
          this.dropDownAlert.alertWithType( "success", "Success", "Signed out successfully", null, 2000 );

          setTimeout( () => {
            this.props.navigation.dispatch(DrawerActions.closeDrawer());
          }, 2500 );
        } )
        .catch( error => {
          console.log( "Error signing out:", error );
          this.dropDownAlert.alertWithType( "error", "Error", "Failed signing out. Please, try again.", null, 2000 );
        } );
    };