GUI: attachments: fix deleting attachments

The page was removed from the view and deleted, but it wasn't removed
from the model's internal list of pages. Therefore the it got deleted a
second time upon model reset leading to a segfault.

src/mkvtoolnix-gui/header_editor/page_model.cpp

@@ -50,6 +50,18 @@ PageModel::appendPage(PageBase *page,
     m_topLevelPages << page;
 }
 
+bool
+PageModel::deletePage(PageBase *page) {
+  auto idx = m_pages.indexOf(page);
+  if (idx == -1)
+    return false;
+
+  m_pages.removeAt(idx);
+  delete page;
+
+  return true;
+}
+
 QList<PageBase *> &
 PageModel::pages() {
   return m_pages;

src/mkvtoolnix-gui/header_editor/page_model.h

@@ -24,6 +24,7 @@ public:
   PageBase *selectedPage(QModelIndex const &idx) const;
 
   void appendPage(PageBase *page, QModelIndex const &parentIdx = {});
+  bool deletePage(PageBase *page);
 
   QList<PageBase *> &pages();
   QList<PageBase *> &topLevelPages();

src/mkvtoolnix-gui/header_editor/tab.cpp

@@ -633,8 +633,7 @@ Tab::removeSelectedAttachment() {
     m_model->removeRow(idx.row(), idx.parent());
 
   m_attachmentsPage->m_children.removeAll(selectedPage);
-
-  delete selectedPage;
+  m_model->deletePage(selectedPage);
 }
 
 KaxAttachedPtr