Fix issue with OnWillRenderObject in HDRP

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderPipeline.cs

         RTHandleSystem.RTHandle         m_DebugFullScreenTempBuffer;
         bool                            m_FullScreenDebugPushed;
         bool                            m_ValidAPI; // False by default mean we render normally, true mean we don't render anything
+        bool                            m_IsCameraRendering; // Use to avoid nested rendering of camera
 
         public Material GetBlitMaterial() { return m_Blit; }
 
             DebugManager.instance.RefreshEditor();
 
             m_ValidAPI = true;
+            m_IsCameraRendering = false;
 
             if (!SetRenderingFeatures())
             {
             if (!m_ValidAPI)
                 return;
 
+            if (m_IsCameraRendering)
+            {
+                Debug.LogWarning("Nested camera rendering is forbidden. If you are calling camera.Render inside OnWillRenderObject callback, use BeginCameraRender callback instead.");
+                return;
+            }
+
             base.Render(renderContext, cameras);
             RenderPipeline.BeginFrameRendering(cameras);
 
                     continue;
 
                 RenderPipeline.BeginCameraRendering(camera);
+                m_IsCameraRendering = true;
 
                 // First, get aggregate of frame settings base on global settings, camera frame settings and debug settings
                 // Note: the SceneView camera will never have additionalCameraData
                     m_DebugScreenSpaceTracingDataArray[0] = data;
                     m_DebugScreenSpaceTracingData.SetData(m_DebugScreenSpaceTracingDataArray);
                 }
+
+                m_IsCameraRendering = false;
             } // For each camera
         }