Correct signature not found error

Hi, I'm trying to get some event triggers working to move between screens. I'm currently getting an error of
"Error Position 12:33. EventHandler "OnTap" with a correct signature not found in type "UniversalMemo.Views.Page.FolderPage"

I've looked around on the net and haven't found anything that helped. I removed the x:class reference to FolderPage but the app then hard crashes on the emulator.

I've cleaned the build many times.
I have a factory reset and rebuild the android emulator multiple times. (I've also done both the above step and this one at the same time)
I think this code may have been working before I did a Visual studio and Xamarin update or 2 ago. I'm not sure what to do to fix this right now other than starting a new project and importing this one into it.

Below is the code

XAML

<?xml version="1.0" encoding="utf-8" ?>

<ContentPage.ToolbarItems>
    <ToolbarItem Text="Add" Clicked="OnTap" />
</ContentPage.ToolbarItems>

...etc...

C#

    protected async void OnTap(object sender, EventArgs e)
    {
       await Navigation.PushModalAsync(new NavigationPage(new NewFolderPage()));
    }