MyApplication_Startup is not on UI Thread

* .NET Core Version:

* Have you experienced this same bug with .NET Framework?:
No

**Problem description:**
Runtime error
System.InvalidOperationException: 'Cross-thread operation not valid: Control 'UserName' accessed from a thread other than the thread it was created on.'

**Expected behavior:**
MyApplication_Startup is on UI Thread as it is on Framework

**Minimal repro:**
```VB
Private Sub MyApplication_Startup(sender As Object, e As StartupEventArgs) Handles Me.Startup
' Get the splash screen.
CType(Me.SplashScreen, SplashScreen1).UserName.Text = "Current user: " & User.Name
End Sub
```