fix: prevent backup thread from posting wx events after shutdown

src/libslic3r/Format/bbs_3mf.cpp

@@ -8218,8 +8218,22 @@ class _BBS_Backup_Manager
     }
 
     void set_post_callback(std::function<void(int)> c) {
-        boost::lock_guard lock(m_mutex);
-        m_post_callback = c;
+        // serialize the swap with in-flight invocation in operator()()
+        {
+            boost::lock_guard post_lock(m_post_callback_mutex);
+            m_post_callback = c;
+        }
+        // catch-up: a task queued while the callback was null missed its
+        // size()==1 notification; re-announce it or backup stalls until exit
+        if (c) {
+            bool pending;
+            {
+                boost::lock_guard lock(m_mutex);
+                pending = !m_ui_tasks.empty();
+            }
+            if (pending)
+                c(0); // outside m_mutex: only this thread drains m_ui_tasks
+        }
     }
 
     void run_ui_tasks() {
@@ -8513,14 +8527,17 @@ class _BBS_Backup_Manager
                     continue;
             }
             m_tasks.pop_front();
-            auto callback = m_post_callback;
             lock.unlock();
             process_task(t);
             lock.lock();
             if (t.type > None) {
                 m_ui_tasks.push_back(t);
-                if (m_ui_tasks.size() == 1 && callback)
-                    callback(0);
+                if (m_ui_tasks.size() == 1) {
+                    // re-read under lock: shutdown may swap in nullptr during process_task()
+                    boost::lock_guard post_lock(m_post_callback_mutex);
+                    if (m_post_callback)
+                        m_post_callback(0);
+                }
             }
         }
     }
@@ -8558,13 +8575,19 @@ class _BBS_Backup_Manager
     size_t m_task_seq = 0;
     // param 0: should call run_ui_tasks
     // param 1: should backup current project
+    // contract: invoked with m_mutex held (worker); must not re-enter this
+    // manager. duplicate param-0 calls are fine: run_ui_tasks() drains the
+    // whole queue, extras are no-ops
     std::function<void(int)> m_post_callback;
     long m_interval = 1 * 60;
     boost::system_time m_next_backup;
     Model m_temp_model; // visit only in main thread
     bool m_other_changes = false; // visit only in main thread
     bool m_other_changes_backup = false; // visit only in main thread
     std::vector<std::pair<ModelObject*, size_t>> m_gaurd_objects;
+    // serializes m_post_callback swap vs worker invocation; lock order: m_mutex -> m_post_callback_mutex
+    // boost primitives to match the class's existing synchronization (m_cond waits need boost locks)
+    boost::mutex m_post_callback_mutex;
     boost::thread m_thread;
 };