If layer URI's aren't usable by plugin, plugin has laggy errors

[HeatherHillers]

In my project, the plugin is unable to create a connection based on the URI's of the layers in the legend. (I don't know why yet, when I do I will make a ticket.)

When the Plugin can't create the connection, it can be very bad for the user, because every click on a legend layer results in a plugin error. Since the plugin error involves trying and failing to run a query, it is also a laggy error. The behavior could be more safe in this state.

I made a pull request with a suggested fix.

[hdus]

@HeatherHillers thanks a lot for your contribution! I have created the new on-off_icon branch for testing and discussing this solution.