OnResume() and OnSleep() called but not working properly - xamarin

Question

I'm a new Xamarin developper and I'm trying to build an app with Xamarin.Forms. Everything works properly on the app but I noticed a strange behaviour : be it on device or with the emulator, when I turn the power off one time then on again, OnSleep() and OnResume() methods work just fine.
But the problem is, when I repeat the same operation a second time, the application freezes and to unlock it I have to either close the app and open it again or go to menu where you can select other apps working in background (I don't know how it's called) and return on the app. I checked with breakpoints and those two methods are called on the first and second time. I tried removing everything from those methods but the problem persists. Does anyone know why it behaves like that ? Thank you.
In my App.xaml.cs :
protected override void OnStart()
{
if (isLoggedIn())
{
MainPage = new NavigationPage(new MDPage());
GeneralViewModel general = new GeneralViewModel();
general.checkUser();
}
else
{
MainPage = new NavigationPage(new LoginPage());
}
}
protected override void OnSleep()
{
// Handle when your app sleeps
}
protected override void OnResume()
{
if (isLoggedIn()) //With a breakpoint, this is executed
{
MainPage.Navigation.PushAsync(new MDPage());
/*this is also executed but the second time,
*even if it is executed the application freezes*/
GeneralViewModel general = new GeneralViewModel();
general.checkUser();
}
else
{
MainPage = new NavigationPage(new LoginPage());
}
}
public bool isLoggedIn()
{
if (Settings.GeneralSettings != null && Settings.GeneralSettings != "")
{
return true;
}
else
{
return false;
}
}
The GeneralViewModel method I call (btw even without it the problem persists)
public void DoSomething()
{
ThreadPool.QueueUserWorkItem(o => checkUser());
}
public Task checkUser()
{
if (Settings.GeneralSettings != null && Settings.GeneralSettings != "")
{
DoSomething();
}
else
{
try
{
var data = Application.Current.MainPage;
Device.BeginInvokeOnMainThread(async () =>
{
await data.DisplayAlert("Information(s)", "Vous avez été déconnecté", "OK");
});
}
catch (Exception e)
{
Console.WriteLine("exception");
}
}
return default;
}
edit : I checked the console and it seems even though nothing appears on the screen, when I press a button on my frozen page the action the button should do happens but the screen doesn't change.