Steps to reproduce:
1. Create a new template project
2. After the project is created immediately navigate to SampleSceneAssets/Materials/General
3. Observe that thumbnails break after a while of default material/prefab icons shown
4. Observe the following error in the console:
[Worker0] NullReferenceException: Object reference not set to an instance of an object
UnityEngine.Rendering.HighDefinition.HDRenderPipelineGlobalSettings.EnsureShadersCompiled () (at Library/PackageCache/com.unity.render-pipelines.high-definition@12.0.0/Runtime/RenderPipeline/HDRenderPipelineGlobalSettings.cs:563)
UnityEngine.Rendering.HighDefinition.HDRenderPipeline.Render (UnityEngine.Rendering.ScriptableRenderContext renderContext, System.Collections.Generic.List`1[T] cameras) (at Library/PackageCache/com.unity.render-pipelines.high-definition@12.0.0/Runtime/RenderPipeline/HDRenderPipeline.cs:1017)
UnityEngine.Rendering.RenderPipeline.InternalRender (UnityEngine.Rendering.ScriptableRenderContext context, System.Collections.Generic.List`1[T] cameras) (at <546c054bece8453b98a8822cb5defeb1>:0)
UnityEngine.Rendering.RenderPipelineManager.DoRenderLoop_Internal (UnityEngine.Rendering.RenderPipelineAsset pipe, System.IntPtr loopPtr, System.Collections.Generic.List`1[T] renderRequests, Unity.Collections.LowLevel.Unsafe.AtomicSafetyHandle safety) (at <546c054bece8453b98a8822cb5defeb1>:0)
UnityEditor.AssetPreviewUpdater:CreatePreviewForAsset(Object, Object[], String)

Expected result: Material and prefab thumbnails should not break on new created project with HDRenderPipelineGlobalSettings.EnsureShadersCompiled

Reproduced in: 2021.2.0a20, 2021.2.0a17
Reproduces in Mac and Windows

Apparently happens on URP too (https://forum.unity.com/threads/prefab-meshes-material-thumbnails-in-project-view-are-shown-as-empty-grey-box.1139461/)