Fix possible use-after-free when closing Blender with File Browser open

I think there wasn't actually any issue currently, but only by luck. We still
passed around and NULL-checked a pointer to freed memory (the file operator,
`SpaceFile.op`) which is easy to break and should be avoided.
Noticed while testing D8598.

source/blender/editors/include/ED_fileselect.h

@@ -35,6 +35,7 @@ struct SpaceFile;
 struct bContext;
 struct bScreen;
 struct uiBlock;
+struct wmOperator;
 struct wmWindow;
 struct wmWindowManager;
 
@@ -145,6 +146,9 @@ void ED_fileselect_window_params_get(const struct wmWindow *win,
                                      int win_size[2],
                                      bool *is_maximized);
 
+struct ScrArea *ED_fileselect_handler_area_find(const struct wmWindow *win,
+                                                const struct wmOperator *file_operator);
+
 int ED_path_extension_type(const char *path);
 int ED_file_extension_icon(const char *path);
 

source/blender/editors/space_file/filesel.c

@@ -61,6 +61,7 @@
 #include "BLF_api.h"
 
 #include "ED_fileselect.h"
+#include "ED_screen.h"
 
 #include "WM_api.h"
 #include "WM_types.h"
@@ -1050,3 +1051,20 @@ void file_params_renamefile_activate(SpaceFile *sfile, FileSelectParams *params)
     params->rename_flag = 0;
   }
 }
+
+ScrArea *ED_fileselect_handler_area_find(const wmWindow *win, const wmOperator *file_operator)
+{
+  bScreen *screen = WM_window_get_active_screen(win);
+
+  ED_screen_areas_iter (win, screen, area) {
+    if (area->spacetype == SPACE_FILE) {
+      SpaceFile *sfile = area->spacedata.first;
+
+      if (sfile->op == file_operator) {
+        return area;
+      }
+    }
+  }
+
+  return NULL;
+}

source/blender/windowmanager/intern/wm_event_system.c

@@ -1731,8 +1731,21 @@ void WM_event_remove_handlers(bContext *C, ListBase *handlers)
     BLI_assert(handler_base->type != 0);
     if (handler_base->type == WM_HANDLER_TYPE_OP) {
       wmEventHandler_Op *handler = (wmEventHandler_Op *)handler_base;
+
       if (handler->op) {
         wmWindow *win = CTX_wm_window(C);
+
+        if (handler->is_fileselect) {
+          /* Exit File Browsers refering to this handler/operator. */
+          LISTBASE_FOREACH (wmWindow *, temp_win, &wm->windows) {
+            ScrArea *file_area = ED_fileselect_handler_area_find(temp_win, handler->op);
+            if (!file_area) {
+              continue;
+            }
+            ED_area_exit(C, file_area);
+          }
+        }
+
         if (handler->op->type->cancel) {
           ScrArea *area = CTX_wm_area(C);
           ARegion *region = CTX_wm_region(C);