Fix crash on open TreeSheets files on MacOS

When file is loaded directly via the open protocol on MacOS,
`System::FileUsed(const wxString&, Document*)` had been called before
the app event loop was entered. It thus tried to access the uninitialized
file system watcher which led to a crash.

Fix this by contructing the file system watcher right away.

src/myapp.h

@@ -165,7 +165,6 @@ struct MyApp : wxApp {
     void OnEventLoopEnter(wxEventLoopBase *WXUNUSED(loop)) {
         if (!initateventloop) {
             initateventloop = true;
-            frame->AppOnEventLoopEnter();
             sys->Init(filename);
         }
     }

src/myframe.h

@@ -5,7 +5,7 @@ struct MyFrame : wxFrame {
     wxTaskBarIcon tbi;
     wxMenu *editmenupopup;
     wxFileHistory filehistory;
-    unique_ptr<wxFileSystemWatcher> watcher {nullptr};
+    unique_ptr<wxFileSystemWatcher> watcher {make_unique<wxFileSystemWatcher>()};
     wxAuiNotebook *nb {nullptr};
     unique_ptr<wxAuiManager> aui {make_unique<wxAuiManager>(this)};
     wxBitmap line_nw;
@@ -710,14 +710,10 @@ struct MyFrame : wxFrame {
 
         SetFileAssoc(exename);
 
-        wxSafeYield();
-    }
-
-    void AppOnEventLoopEnter() {
-        // Have to do this here, if we do it in the Frame constructor above, it crashes on OS X.
-        watcher.reset(new wxFileSystemWatcher());
         watcher->SetOwner(this);
         Connect(wxEVT_FSWATCHER, wxFileSystemWatcherEventHandler(MyFrame::OnFileSystemEvent));
+
+        wxSafeYield();
     }
 
     ~MyFrame() {