Added msg.content null check

src/Discord.Net.Commands/Extensions/MessageExtensions.cs

@@ -32,7 +32,7 @@ namespace Discord.Commands
         public static bool HasStringPrefix(this IUserMessage msg, string str, ref int argPos, StringComparison comparisonType = StringComparison.Ordinal)
         {
             var text = msg.Content;
-            if (text.StartsWith(str, comparisonType))
+            if (!string.IsNullOrEmpty(text) && text.StartsWith(str, comparisonType))
             {
                 argPos = str.Length;
                 return true;