Describe the bug

In the Microsoft.MixedReality.Toolkit.Utilities.CameraCache there is a loophole to fall through, if cachedCamera and Camera.main are both null.

The code reads (l. 36, ff.):

if (mainCamera == null)
{
    Debug.Log("No main camera found. Searching for cameras in the scene.");

    // If no main camera was found, try to determine one.
    Camera[] cameras = GameObject.FindObjectsOfType<Camera>();
    if (cameras.Length == 0)
    {
        Debug.LogWarning("No cameras found. Creating a \"MainCamera\".");
        mainCamera = new GameObject("Main Camera", typeof(Camera), typeof(AudioListener)) { tag = "MainCamera" }.GetComponent<Camera>();
    }
    else
    {
        Debug.LogWarning("The Mixed Reality Toolkit requires one camera in the scene to be tagged as \"MainCamera\". Please ensure the application's main camera is tagged.");
    }
}

So if Cameras are found, they are never assigned to the cameraCache.

To reproduce

Steps to reproduce the behavior:

1. Create an app with no MRTK Profile assigned to the MRTK
2. Assign an MRTK profile during runtime
   --> Observe that CameraCache.Main returns null
   --> Observe that it can never access the cameras found in the scene

Expected behavior

The MRTK can be assigned a profile and initialized AFTER preparing the scene

Your setup (please complete the following information)

• Unity Version [e.g. 2019.4.21f1]
• MRTK Version [e.g. v2.5.4]

Target platform (please complete the following information)

• HoloLens
• HoloLens 2
• Unity Editor

Additional context

Add any other context about the problem here.
I solved this by changing the CameraCache.cs script. But of course, I would rather not change scripts from the MRTK package