Fix NullReferenceException when HttpContext is null

This method will throw a NullReferenceException in contexts without an HttpContext, like is the case in background threads. As HttpContext is only used to set an isDebug flag, it seems more sensible to not throw in cases without an HttpContext, but simply set isDebug to false instead. If this is not acceptable behavior, could we perhaps look at something other than HttpContext.Current.IsDebuggingEnabled to decide whether isDebug should be set?

src/Umbraco.Core/PropertyEditors/ValueConverters/GridValueConverter.cs

@@ -40,12 +40,13 @@ namespace Umbraco.Core.PropertyEditors.ValueConverters
 
                     //TODO: Change all singleton access to use ctor injection in v8!!!
                     //TODO: That would mean that property value converters would need to be request lifespan, hrm....
+                    bool isDebug = HttpContext.Current != null && HttpContext.Current.IsDebuggingEnabled;
                     var gridConfig = UmbracoConfig.For.GridConfig(
                         ApplicationContext.Current.ProfilingLogger.Logger,
                         ApplicationContext.Current.ApplicationCache.RuntimeCache,
                         new DirectoryInfo(IOHelper.MapPath(SystemDirectories.AppPlugins)),
                         new DirectoryInfo(IOHelper.MapPath(SystemDirectories.Config)),
-                        HttpContext.Current.IsDebuggingEnabled);
+                        isDebug);
                     
                     var sections = GetArray(obj, "sections");
                     foreach (var section in sections.Cast<JObject>())
@@ -112,4 +113,4 @@ namespace Umbraco.Core.PropertyEditors.ValueConverters
         }
 
     }
-}
+}