XamlParseException when using the XAML Designer - xaml

Question

Important: Consider every tag in the .xaml codes below closed
I've searched everywhere for the answer but none of the questions in this website (or the MSDN website) solved it. Everytime I create a new Windows Phone 8.1 project I get the XamlParseException, even when I choose the Blank App project. I will list everything I tried.
Reinstall VS 2015
Reinstall VS 2015 without the SDK and then download it later with the emulators
Enable Developer Mode on Windows
Disable Project Code
Open the .xaml files with the Automatic Editor Selector and XAML Designer
Enable XAML Designer in the Options, of course
Update Windows 10 (note: I couldn't upgrade to 1809, I'm stuck with 1803)
Enable Hypervisor on the BIOS
Reboot computer
Adding the 'ThemeResources.xaml' manually on the project
Build, Rebuild and Clean the solution
Using different .NET Framework versions
Here is my App.xaml code:
<Application
x:Class="App2.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App2">
Here is my MainPage.xaml code:
<Page
x:Class="App2.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App2"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<Grid>
</Grid>

Answer 1

I got it to work just by updating to Windows 10 1089, don't forget to enable Developer Mode once you upgrade, since "Sideload Apps" isn't enough.