Avoid potential race condition

In theory, if "ClearFormCache" is called after we check `contains` but
before we execute the `return` line, we could get an exception here.

If we're concerned about performance here, we could consider switching
to the ConcurrentDictionary.

Author: haacked

src/GitHub.Api/SimpleApiClientFactory.cs

@@ -1,10 +1,10 @@
 using System;
+using System.Collections.Generic;
 using System.ComponentModel.Composition;
 using GitHub.Models;
 using GitHub.Primitives;
 using GitHub.Services;
 using Octokit;
-using System.Collections.Generic;
 
 namespace GitHub.Api
 {
@@ -28,10 +28,6 @@ public SimpleApiClientFactory(IProgram program, Lazy<IEnterpriseProbeTask> enter
 
         public ISimpleApiClient Create(UriString repositoryUrl)
         {
-            var contains = cache.ContainsKey(repositoryUrl);
-            if (contains)
-                return cache[repositoryUrl];
-
             lock (cache)
             {
                 if (!cache.ContainsKey(repositoryUrl))