When a single coded step is selected and run it fails with the following exception:
System.ArgumentNullException: Start reference element can't be null. Please make sure the DOM is loaded by either calling NavigateTo() to a page or add browser RefreshDomTree() prior to the code which needs the DOM.
This happens because a single coded step does not call ActiveBrowser.RefreshDomTree() call as it needs the DOM to execute against. We handle that internally for Navigate or non-coded steps with elements because we can figure out we need the DOM.
You should solve this issue by adding ActiveBrowser.RefreshDomTree() as the first row in the coded step.