[Ide] Fix TypeInitializationException if no templates available

The new templating engine used for .NET Core projects throws an
exception if an attempt is made to save the templating cache when
there are no templates available. This then causes the MonoDevelop.Ide
addin to not load due a TypeInitializationException being thrown
when creating the MicrosoftTemplateEngineProjectTemplatingProvider.

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
  ---> System.TypeInitializationException: The type initializer for
'MonoDevelop.Ide.Templates.MicrosoftTemplateEngineProjectTemplatingProvider' threw an exception.
  ---> System.NullReferenceException: Object reference not set to an instance of an object
  at Microsoft.TemplateEngine.Edge.Settings.SettingsLoader.Save (Microsoft.TemplateEngine.Edge.Settings.TemplateCache cacheToSave) in
  at Microsoft.TemplateEngine.Edge.Settings.SettingsLoader.Save ()
  at MonoDevelop.Ide.Templates.MicrosoftTemplateEngineProjectTemplatingProvider.UpdateCache () in
  main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Templates/MicrosoftTemplateEngineProjectTemplatingProvider.cs:85
  at MonoDevelop.Ide.Templates.MicrosoftTemplateEngineProjectTemplatingProvider..cctor () in
  main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Templates/MicrosoftTemplateEngineProjectTemplatingProvider.cs:66

Now if there are no templates then no attempt is made to update the
templating cache which avoids this exception and prevents extra
work being done when it is not needed. If a template becomes available
the cache will be updated.

1 files changed, 7 insertions, 0 deletions

diff --git a/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Templates/MicrosoftTemplateEngineProjectTemplatingProvider.cs b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Templates/MicrosoftTemplateEngineProjectTemplatingProvider.cs
index e34ac65025..f5f75c32e6 100644
--- a/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Templates/MicrosoftTemplateEngineProjectTemplatingProvider.cs
+++ b/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Templates/MicrosoftTemplateEngineProjectTemplatingProvider.cs
@@ -73,6 +73,13 @@ namespace MonoDevelop.Ide.Templates
 		{
 			if (dontUpdateCache)//Avoid updating cache while scan paths are added during registration 
 				return;
+
+			// Prevent a TypeInitializationException in when calling SettingsLoader.Save when no templates
+			// are available, which throws an exception, by returning here. This prevents the MonoDevelop.Ide addin
+			// from loading. In practice this should not happen unless the .NET Core addin is disabled.
+			if (!TemplatesNodes.Any ())
+				return;
+
 			var paths = new Paths (environmentSettings);
 
 			//TODO: Uncomment this IF, but also add logic to invalidate/check if new templates were added from newly installed AddOns...