fix(ui): Prevent suicide crash on logout

Fixes #2480

The GUI was deleting itself on logout, but was doing while deep inside the middle of GUI code, resulting in a series of use-after-free on the return path. We now freeze the GUI and logout asynchronously in a more suitable call stack.

src/nexus.cpp

@@ -258,6 +258,12 @@ bool Nexus::tryRemoveFile(const QString& filepath)
     return writable;
 }
 
+void Nexus::showLoginLater()
+{
+    GUI::setEnabled(false);
+    QMetaObject::invokeMethod(&getInstance(), "showLogin", Qt::QueuedConnection);
+}
+
 #ifdef Q_OS_MAC
 void Nexus::retranslateUi()
 {

src/nexus.h

@@ -45,7 +45,6 @@ class Nexus : public QObject
     Q_OBJECT
 public:
     void start(); ///< Sets up invariants and calls showLogin
-    void showLogin(); ///< Hides the man GUI, delete the profile, and shows the login screen
     /// Hides the login screen and shows the GUI for the given profile.
     /// Will delete the current GUI, if it exists.
     void showMainGUI();
@@ -59,7 +58,12 @@ public:
     static QString getSupportedImageFilter();
     static bool tryRemoveFile(const QString& filepath); ///< Dangerous way to find out if a path is writable
 
+public slots:
+    void showLogin(); ///< Hides the man GUI, delete the profile, and shows the login screen
+    void showLoginLater(); ///< Calls showLogin asynchronously, so we can safely logout from within the main GUI
+
 #ifdef Q_OS_MAC
+public:
     QMenuBar* globalMenuBar;
     QMenu* viewMenu;
     QMenu* windowMenu;

src/widget/form/profileform.cpp

@@ -380,7 +380,7 @@ void ProfileForm::onDeleteClicked()
             GUI::showError(tr("Files could not be deleted!", "deletion failed title"), message);
         }
 
-        nexus.showLogin();
+        nexus.showLoginLater();
     }
 }
 
@@ -388,7 +388,7 @@ void ProfileForm::onLogoutClicked()
 {
     Nexus& nexus = Nexus::getInstance();
     Settings::getInstance().saveGlobal();
-    nexus.showLogin();
+    nexus.showLoginLater();
 }
 
 void ProfileForm::setPasswordButtonsText()