Fix uninitialised flag in TaskChain

Fixes #350 (or part of it). If a task chain had been aborted because of a
missing IP setting, pressing the back button would cause the tasks up to the
prohibited one to be displayed.

tablet_qt/tasklib/taskchain.cpp

@@ -38,7 +38,8 @@ TaskChain::TaskChain(CamcopsApp& app,
     m_creation_method(creation_method),
     m_title(title),
     m_subtitle(subtitle),
-    m_current_task_index(-1)
+    m_current_task_index(-1),
+    m_proceed_when_app_has_closed_last_task(false)
 {
     QObject::connect(&m_app, &CamcopsApp::subWindowFinishedClosing,
                      this, &TaskChain::onAppSubWindowClosed);