Undefined index: source_provider migrate_plugins.module:98

The module needs to be updated to work with migrate_plus and migrate_tools version 4.x because there is no longer a "source_provider" in the plugin definition.

From inspecting the output of the pluginDefinition I see this structure:

array(4) {
  ["requirements_met"]=>
  bool(true)
  ["id"]=>
  string(3) "url"
  ["class"]=>
  string(45) "Drupal\migrate_plus\Plugin\migrate\source\Url"
  ["provider"]=>
  array(3) {
    [0]=>
    string(12) "migrate_plus"
    [2]=>
    string(7) "migrate"
    [3]=>
    string(4) "core"
  }
}