Robustness and warning fixes.

src/gui/main.cc

@@ -122,9 +122,16 @@ bool FluxEngineApp::IsWorkerThreadRunning() const
 
 void FluxEngineApp::OnExec(const ExecEvent& event)
 {
+    try
+    {
         event.RunCallback();
         if (event.IsSynchronous())
             execSemaphore.Post();
+    }
+    catch (std::exception& e)
+    {
+        std::cerr << "Unhandled exception: " << e.what() << "\n";
+    }
 }
 
 void runOnUiThread(std::function<void()> callback)

src/gui/mainwindow.cc

@@ -481,6 +481,9 @@ public:
             visualiser->Clear();
             _filesystemModel->Clear(Path());
             _currentDisk = nullptr;
+            _filesystemCapabilities = 0;
+            _filesystemIsReadOnly = true;
+            _filesystemNeedsFlushing = false;
 
             _state = STATE_BROWSING_WORKING;
             UpdateState();