Missing argument 2 for myplugin_widget

Those error messages don’t appear to be related to this plugin since they’re being generated by code in your theme. My best guess is that you’ll need to modify the call to add_filter() in your theme to make sure all 3 arguments get passed to your callback function.

You’ll probably have a line in your theme that looks something like this:

add_filter( 'widget_title', 'myplugin_widget_title' );

It should look like this:

add_filter( 'widget_title', 'myplugin_widget_title', 10, 3 );

You can also update the parameters in your callback to make them optional:

function myplugin_widget_title( $title, $instance = array(), $id_base = '' ) {}