Fix issue with preprocessor

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/BuildPlayer/HDRPCustomBuildProcessor.cs

 using UnityEditor.Build;
 using UnityEditor.Build.Reporting;
 using UnityEngine;
+using UnityEngine.Experimental.Rendering;
+using UnityEngine.Experimental.Rendering.HDPipeline;
 
 class HDRPCustomBuildProcessor : IPreprocessBuildWithReport
 {
     {
+        // Don't execute the preprocess if we are not HDRenderPipeline
+        HDRenderPipeline hdrp = RenderPipelineManager.currentPipeline as HDRenderPipeline;
+        if (hdrp == null)
+            return;
+
         // Note: If you add new platform in this function, think about adding support in IsSupportedPlatform() function in HDRenderPipeline.cs
 
         // If platform is supported all good

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/BuildPlayer/HDRPVariantStripper.cs

         public int callbackOrder { get { return 0; } }
         public void OnProcessShader(Shader shader, ShaderSnippetData snippet, IList<ShaderCompilerData> inputData)
         {
+            // This test will also return if we are not using HDRenderPipelineAsset
             if (m_CurrentHDRPAsset == null || !m_CurrentHDRPAsset.allowShaderVariantStripping)
                 return;
 

ScriptableRenderPipeline/LightweightPipeline/LWRP/Editor/ShaderPreprocessor.cs

 
         public void OnProcessShader(Shader shader, ShaderSnippetData snippetData, IList<ShaderCompilerData> compilerDataList)
         {
+            LightweightRP lw = RenderPipelineManager.currentPipeline as LightweightRP;
+            if (lw == null)
+                return;
+
             PipelineCapabilities capabilities = LightweightRP.GetPipelineCapabilities();
             int prevVariantCount = compilerDataList.Count;