fix a use-after-free during widget teardown

Author: jakoch

include/fifechan/widget.hpp

@@ -984,6 +984,14 @@ namespace fcn
              */
             virtual FocusHandler* _getFocusHandler();
 
+            /**
+             * Detaches the given focus handler from every live widget.
+             *
+             * Call this before destroying a focus handler so widgets that
+             * outlive it never dereference a dangling pointer.
+             */
+            static void _detachFocusHandlerFromAll(FocusHandler* focusHandler);
+
             /**
              * Adds an action listener to the widget. When an action event
              * is fired by the widget the action listeners of the widget

src/gui.cpp

@@ -101,6 +101,7 @@ namespace fcn
             }
             mTop = nullptr;
         }
+        Widget::_detachFocusHandlerFromAll(mFocusHandler);
         Widget::_setVisibilityEventHandler(nullptr);
         Widget::_setGuiDeathListener(nullptr);
 

src/widget.cpp

@@ -860,6 +860,15 @@ namespace fcn
         return mFocusHandler;
     }
 
+    void Widget::_detachFocusHandlerFromAll(FocusHandler* focusHandler)
+    {
+        for (auto* widget : widgetInstancesRegistry()) {
+            if (widget != nullptr && widget->mFocusHandler == focusHandler) {
+                widget->mFocusHandler = nullptr;
+            }
+        }
+    }
+
     void Widget::_setVisibilityEventHandler(VisibilityEventHandler* visibilityEventHandler)
     {
         mVisibilityEventHandler = visibilityEventHandler;