Rethink your strategy of storing your data. If you need eval to access your variable something is going wrong. In case Stephen doesn't chime in: read this.
As for your problem: in a GUI you should always use an explicit parent object. Once you have made sure of that, this code should cause your legend to fill automatically, but only if it already exists. If adding the parent doesn't solve your question you should try to make a MWE so we can run your code without any other dependencies and can reproduce your issue.
