Fixed C# and CLI backends to check for null values when marshaling classes.

src/Generator/Generators/CLI/CLIMarshal.cs

@@ -235,7 +235,8 @@ namespace CppSharp.Generators.CLI
         public void WriteClassInstance(Class @class, string instance)
         {
             if (@class.IsRefType)
-                Context.Return.Write("gcnew ");
+                Context.Return.Write("({0} == nullptr) ? nullptr : gcnew ",
+                    instance);
 
             Context.Return.Write("{0}(", QualifiedIdentifier(@class));
             Context.Return.Write("(::{0}*)", @class.QualifiedOriginalName);

src/Generator/Generators/CSharp/CSharpMarshal.cs

@@ -294,6 +294,10 @@ namespace CppSharp.Generators.CSharp
                 instance = instanceName;
             }
 
+            if (@class.IsRefType)
+                Context.Return.Write("({0} == IntPtr.Zero) ? null : ",
+                    instance);
+
             Context.Return.Write("new {0}({1})",
                 QualifiedIdentifier(@class.OriginalClass ?? @class) +
                 (Context.Driver.Options.GenerateAbstractImpls && @class.IsAbstract ?