Steps to reproduce:
- Unzip the player from link in comments.
- Open "HDTemplate-srp-tarball.exe"
- Press spacebar to create a decal projector through script.

Result: Application freeze, with following errors :

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ArgumentNullException: Value cannot be null.
Parameter name: sourceArray
at System.Array.Copy (System.Array sourceArray, System.Array destinationArray, System.Int32 length) [0x00003] in <c79628fadf574d3a8feae0871fad28ef>:0
at UnityEngine.Rendering.HighDefinition.DecalSystem+DecalSet.SetCullResult (UnityEngine.Rendering.HighDefinition.DecalSystem+CullResult+Set value) [0x00033] in H:\Templates\HDTemplate-srp-tarball\Library\PackageCache\com.unity.render-pipelines.high-definition@53c7c50d74a6-1579085591005\Runtime\Material\Decal\DecalSystem.cs:846
at UnityEngine.Rendering.HighDefinition.DecalSystem.LoadCullResults (UnityEngine.Rendering.HighDefinition.DecalSystem+CullResult cullResult) [0x00036] in H:\Templates\HDTemplate-srp-tarball\Library\PackageCache\com.unity.render-pipelines.high-definition@53c7c50d74a6-1579085591005\Runtime\Material\Decal\DecalSystem.cs:1130
at UnityEngine.Rendering.HighDefinition.HDRenderPipeline.ExecuteRenderRequest (UnityEngine.Rendering.HighDefinition.HDRenderPipeline+RenderRequest renderRequest, UnityEngine.Rendering.ScriptableRenderContext renderContext, UnityEngine.Rendering.CommandBuffer cmd, UnityEngine.Rendering.HighDefinition.AOVRequestData aovRequest) [0x00183] in H:\Templates\HDTemplate-srp-tarball\Library\PackageCache\com.unity.render-pipelines.high-definition@53c7c50d74a6-1579085591005\Runtime\RenderPipeline\HDRenderPipeline.cs:1848
at UnityEngine.Rendering.HighDefinition.HDRenderPipeline.Render (UnityEngine.Rendering.ScriptableRenderContext renderContext, UnityEngine.Camera[] cameras) [0x00c2e] in H:\Templates\HDTemplate-srp-tarball\Library\PackageCache\com.unity.render-pipelines.high-definition@53c7c50d74a6-1579085591005\Runtime\RenderPipeline\HDRenderPipeline.cs:1723
at UnityEngine.Rendering.RenderPipeline.InternalRender (UnityEngine.Rendering.ScriptableRenderContext context, UnityEngine.Camera[] cameras) [0x0001c] in <0bc11320470b45a397aa1779b1da406c>:0
at UnityEngine.Rendering.RenderPipelineManager.DoRenderLoop_Internal (UnityEngine.Rendering.RenderPipelineAsset pipe, System.IntPtr loopPtr) [0x0003d] in <0bc11320470b45a397aa1779b1da406c>:0

(Filename: <c79628fadf574d3a8feae0871fad28ef> Line: 0)

XRSystem.ReleaseFrame() was not called!
UnityEngine.DebugLogHandler:Internal_Log(LogType, LogOption, String, Object)
UnityEngine.DebugLogHandler:LogFormat(LogType, Object, String, Object[])
UnityEngine.Logger:Log(LogType, Object)
UnityEngine.Debug:LogWarning(Object)
UnityEngine.Rendering.HighDefinition.XRSystem:SetupFrame(Camera[], Boolean, Boolean) (at H:\Templates\HDTemplate-srp-tarball\Library\PackageCache\com.unity.render-pipelines.high-definition@53c7c50d74a6-1579085591005\Runtime\RenderPipeline\XR\XRSystem.cs:114)
UnityEngine.Rendering.HighDefinition.HDRenderPipeline:Render(ScriptableRenderContext, Camera[]) (at H:\Templates\HDTemplate-srp-tarball\Library\PackageCache\com.unity.render-pipelines.high-definition@53c7c50d74a6-1579085591005\Runtime\RenderPipeline\HDRenderPipeline.cs:1206)
UnityEngine.Rendering.RenderPipeline:InternalRender(ScriptableRenderContext, Camera[])
UnityEngine.Rendering.RenderPipelineManager:DoRenderLoop_Internal(RenderPipelineAsset, IntPtr)

(Filename: H:/Templates/HDTemplate-srp-tarball/Library/PackageCache/com.unity.render-pipelines.high-definition@53c7c50d74a6-1579085591005/Runtime/RenderPipeline/XR/XRSystem.cs Line: 114)

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Expected : A decal projector should be created and rendered, displaying the "3D noise" texture.

Reproduces with :
- Unity 2019.3.0f5 / HDRP 7.1.7
- Unity 2019.3.0f6 (3805be3cb06a) / HDRP 7.2.0 (74e716d78553d6ab19142f17f38903b6b6c54521)

Notes:
- Import the attached "RuntimeDecals.unitypackage" file to see the scene setup.
- Holding the s key while pressing spacebar will create the decal projector in a safe manner : the gameobject is disabled before adding the component, and re-enabled after assigning a proper decal material.
- I highly suspect that it is caused byt the decal projector default material that doesn't use the HDRP/decal shader. Suggestions : Change the default material assigned to the decal material to use the proper shader, and add a safeguard to avoid breaking the rendering if the assigner shader doesn't suit to decals.