fail fast when extension manager is null

made us to crash right away when extension manager is null to get better info on the situation.

if extension manager is null, vs is seriously broken.

fix https://devdiv.visualstudio.com/DevDiv/_workitems?path=Assigned%20to%20me&_a=query

src/VisualStudio/Core/Def/Implementation/Diagnostics/VisualStudioWorkspaceDiagnosticAnalyzerProviderService.cs

@@ -43,6 +43,12 @@ public VisualStudioWorkspaceDiagnosticAnalyzerProviderService(VisualStudioWorksp
 
             // Get the analyzer assets for installed VSIX extensions through the VSIX extension manager.
             var extensionManager = workspace.GetVsService(assembly.GetType("Microsoft.VisualStudio.ExtensionManager.SVsExtensionManager"));
+            if (extensionManager == null)
+            {
+                // extension manager can't be null. if it is null, then VS is seriously broken.
+                // fail fast right away
+                FailFast.OnFatalException(new Exception("extension manager can't be null"));
+            }
 
             _hostDiagnosticAnalyzerInfo = GetHostAnalyzerPackagesWithName(extensionManager, assembly.GetType("Microsoft.VisualStudio.ExtensionManager.IExtensionContent"));
         }