‘Attempt to modify property of non-object’ warning

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Some simple php, check if it’s set before trying to unset it.

function my_filter_plugin_updates( $value ) {
   if( isset( $value->response['duplicator/duplicator.php'] ) )
       unset( $value->response['duplicator/duplicator.php'] ); //Duplicator

   return $value;
}
add_filter( 'site_transient_update_plugins', 'my_filter_plugin_updates' );