Fix the null reference exception which occurs when DependencyResolver…

….Current.GetService<IConfigurationSource>() returns null.

And just in case, also eliminate the possibility of null refs caused by something setting DependencyResolver.Current to null.

PoliteCaptcha/ReCaptchaGenerator.cs

@@ -12,7 +12,7 @@ namespace PoliteCaptcha
     /// </summary>
     public class ReCaptchaGenerator : ICaptchaGenerator
     {
-        readonly IConfigurationSource configSource;
+        readonly IConfigurationSource _configSource;
 
         public ReCaptchaGenerator()
             : this(new DefaultConfigurationSource())
@@ -21,7 +21,7 @@ public ReCaptchaGenerator()
 
         public ReCaptchaGenerator(IConfigurationSource configSource)
         {
-            this.configSource = configSource;
+            _configSource = configSource;
         }
 
         /// <summary>
@@ -31,15 +31,16 @@ public ReCaptchaGenerator(IConfigurationSource configSource)
         /// <param name="fallbackMessage">An optional message to display above the CAPTCHA when it is displayed as a fallback.</param>
         /// <returns>The reCAPTCHA HTML.</returns>
         public IHtmlString Generate(
-            HtmlHelper htmlHelper, 
+            HtmlHelper htmlHelper,
             string fallbackMessage = null)
         {
             if (htmlHelper == null)
+            {
                 throw new ArgumentNullException("htmlHelper");
+            }
 
-            var configurationSource = DependencyResolver.Current.GetService<IConfigurationSource>();
-
-            var publicApiKey = configSource.GetConfigurationValue(Const.ReCaptchaPublicKeyAppSettingKey);
+            IConfigurationSource configurationSource = GetConfigurationSource();
+            var publicApiKey = configurationSource.GetConfigurationValue(Const.ReCaptchaPublicKeyAppSettingKey);
             if (publicApiKey == null)
             {
                 if (!htmlHelper.ViewContext.HttpContext.Request.IsLocal)
@@ -68,9 +69,28 @@ public ReCaptchaGenerator(IConfigurationSource configSource)
             recaptchaControl.RenderControl(htmlWriter);
 
             var captchaHtml = htmlWriter.InnerWriter.ToString();
-            var template = @"<div class=""PoliteCaptcha editor-field""><span class=""field-validation-error"" data-valmsg-for=""PoliteCaptcha""><span htmlfor=""PoliteCaptcha"">{0}</span></span>{1}</div>";
-
+
+            const string template = @"<div class=""PoliteCaptcha editor-field""><span class=""field-validation-error"" data-valmsg-for=""PoliteCaptcha""><span htmlfor=""PoliteCaptcha"">{0}</span></span>{1}</div>";
             return new MvcHtmlString(string.Format(template, fallbackMessage ?? Const.DefaulFallbackMessage, captchaHtml));  
         }
+
+        private IConfigurationSource GetConfigurationSource()
+        {
+            if (_configSource != null)
+            {
+                return _configSource;
+            }
+
+            if (DependencyResolver.Current != null)
+            {
+                var resolvedConfigSource = DependencyResolver.Current.GetService<IConfigurationSource>();
+                if (resolvedConfigSource != null)
+                {
+                    return resolvedConfigSource;
+                }
+            }
+
+            return new DefaultConfigurationSource();
+        }
     }
 }