Bug when using convention and MapFallbackToPage

### Describe the bug
You need to create IPageRouteModelConvention
In Apply add new template as combination with prefix and template
Use MapFallbackToPage() in startup

### To Reproduce
Copy and run this code
https://github.com/ParadiseFallen/aspnet-ErrorExample

### Exceptions (if any)
nvalidOperationException: Using ExpandEndpoint requires that the replaced endpoint have a unique priority. The following endpoints were found with the same priority:
/_Host
/_Host

### Further technical details
- ASP.NET Core 5
- Пакет SDK для .NET (отражающий любой global.json):
Version: 5.0.301
Commit: ef17233f86

Среда выполнения:
OS Name: Windows
OS Version: 10.0.19042
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.301\

Host (useful for support):
Version: 5.0.7
Commit: 556582d964

.NET SDKs installed:
5.0.301 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download
- Visual Studio 2019