Fix exit crash - develop

plugins/producer_plugin/producer_plugin.cpp

@@ -913,9 +913,8 @@ void producer_plugin::plugin_shutdown() {
    if( my->_thread_pool ) {
       my->_thread_pool->stop();
    }
-   my->_accepted_block_connection.reset();
-   my->_accepted_block_header_connection.reset();
-   my->_irreversible_block_connection.reset();
+
+   app().post( 0, [me = my](){} ); // keep my pointer alive until queue is drained
 }
 
 void producer_plugin::handle_sighup() {