Workaround abstract implementations being generated even if there's no valid base class.

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -1899,7 +1899,9 @@ namespace CppSharp.Generators.CSharp
                 PopBlock(NewLineKind.BeforeNextBlock);
             }
 
-            string className = @class.IsAbstractImpl ? @class.BaseClass.Name : @class.Name;
+            // TODO: We can still get an abstract impl even if BaseClass is null.
+            string className = @class.IsAbstractImpl && @class.BaseClass != null ?
+                @class.BaseClass.Name : @class.Name;
 
             var ctorCall = string.Format("{0}{1}", @class.Name, @class.IsAbstract ? "Internal" : "");
             if (!@class.IsAbstractImpl)