The short answer about that error is that it's an error that announce you that one of the GUI element of the editor is unable to access whatever it tries to access.
This is not your fault, but more like a tiny synchronization problem between the editor and its references. That error can happen if you got some hidden/unfocused active menu (like, often, being a 2nd inspector) that refers (meaning would display if active/focused) an old references that has some changes.
This kind of error can easily be fixed by saving (if not already done) your editor layout (top-right dropdown button called "Layout", then "save Layout"), then (in the same drop down) "Reset Layout" and then you can load the layout back in. This will force the editor to reset all active menu with a selection, hence fixes whatever is wrongly selected a NULL object.
This error can happen if, for example, you have 2 inspectors in your editor (quite useful if you want to keep 1 object selected to access it scripts or components and select stuff from the Project or Hierarchy menus). Sometimes, Unity fails to clean its selection cache when it should be cleaned.
Then, when you press "Play", the editor is attempting to store and cache all current inspector/object/component states (so that it can return to that state once Play is stopped), but it fails to access (and cache) whatever is still wrongly cached to a certain menu (like a 2nd inspector) so it returns that error telling you that some editor property are returning a NullReferenceException (meaning that it doesn't exist).