How to reproduce:
1. Open the attached "Repro_1131416.zip" project
2. In Build Settings, switch the Target Platform to "UWP"
3. Open "Assets/_Scenes/Main" scene
4. Enter Play mode
5. In Hierarchy, select "Main/MixedRealityPlayspace/Main Camera/UIRaycastCamera"
6. In Inspector, enable and disable Camera component
7. Leave Play mode
8. Observe the "GetCurrentHolographicCameraState: No culling frustum available. Culling data may be stale." warning
9. Enter Play mode again
-- Various errors are thrown (full traces below)

Reproducible with - 2018.3.0b12, 2018.3.11f1, 2019.1.0b10, 2019.2.0a10
Could not be tested on 2017.4, 2019.1, 2019.2 (with the attached project) due to a variety of compatibility errors.

• - with a different project

Errors thrown:
1. "Assertion failed on expression: 'IsMatrixValid(matrix)'"
2. "Assertion failed on expression: 'm_FrameIndexGfxThread != m_FrameAddIndex'"
3. "Main camera does not exist!
UnityEngine.XR.WSA.HolographicSettings:get_IsDisplayOpaque()
Microsoft.MixedReality.Toolkit.Core.Definitions.MixedRealityCameraProfile:get_IsOpaque() (at Assets/MixedRealityToolkit/Definitions/MixedRealityCameraProfile.cs:78)
Microsoft.MixedReality.Toolkit.Core.Services.MixedRealityToolkit:InitializeServiceLocator() (at Assets/MixedRealityToolkit/Services/MixedRealityToolkit.cs:178)
Microsoft.MixedReality.Toolkit.Core.Services.MixedRealityToolkit:InitializeInstance() (at Assets/MixedRealityToolkit/Services/MixedRealityToolkit.cs:443)
Microsoft.MixedReality.Toolkit.Core.Services.MixedRealityToolkit:Awake() (at Assets/MixedRealityToolkit/Services/MixedRealityToolkit.cs:555)
"