Fix null reference access when ContentType is null (#2501)

Octokit.Tests/Exceptions/ApiExceptionTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Net;
 #if !NO_SERIALIZABLE
@@ -141,6 +142,16 @@ namespace Octokit.Tests.Exceptions
                 Assert.NotNull(stringRepresentation);
             }
 
+            [Fact]
+            public void DoesNotThrowIfContentTypeIsNotDefined()
+            {
+                var response = CreateResponse(HttpStatusCode.GatewayTimeout, null, new Dictionary<string, string>(), null);
+
+                var exception = new ApiException(response);
+                var stringRepresentation = exception.ToString();
+                Assert.NotNull(stringRepresentation);
+            }
+
             [Fact]
             public void DoesNotPrintImageContent()
             {

Octokit/Exceptions/ApiException.cs

@@ -200,7 +200,7 @@ namespace Octokit
         {
             get
             {
-                return HttpResponse != null
+                return HttpResponse?.ContentType != null
                        && !HttpResponse.ContentType.StartsWith("image/", StringComparison.OrdinalIgnoreCase)
                        && HttpResponse.Body is string
                     ? (string)HttpResponse.Body : string.Empty;