Null class property can only be retrieved once

[stuffaboutpete]

When accessing a class property which has no default value and has not been assigned a value, the first retrieval works just fine but any subsequent 'get' triggers an error indicating that null is not a valid value.

I believe the fix is below, but I haven't run it in any tests yet...

src/Registry/Member.js

-169        classInstanceData.properties[name] = defaultValue;
+169        if (defaultValue !== null) classInstanceData.properties[name] = defaultValue;