Clear ATCDialog instance on reset

Showed up as a variety of crashes on Sentry, due
to a stale instance of this object hanging around
after reset.

Sentry-Id:FLIGHTGEAR-T5

src/ATC/atcdialog.cxx

@@ -214,6 +214,14 @@ void FGATCDialogNew::init() {
     globals->get_props()->setIntValue("/sim/atc/transmission-num", -1);
 }
 
+void FGATCDialogNew::shutdown()
+{
+    auto commands = globals->get_commands();
+    commands->removeCommand("ATC-dialog");
+    commands->removeCommand("ATC-freq-search");
+    commands->removeCommand("ATC-freq-display");
+}
+
 // Display the ATC popup dialog box with options relevant to the users current situation.
 //
 // So, the first thing we need to do is check the frequency that our pilot's nav radio
@@ -242,6 +250,15 @@ void FGATCDialogNew::PopupDialog() {
     return;
 }
 
+void FGATCDialogNew::hackyReset()
+{
+    if (_instance) {
+        _instance->shutdown();
+        delete _instance;
+        _instance = nullptr;
+    }
+}
+
 void FGATCDialogNew::update(double dt) {
 //  double onBoardRadioFreq0 =
 //      fgGetDouble("/instrumentation/comm[0]/frequencies/selected-mhz");

src/ATC/atcdialog.hxx

@@ -35,6 +35,11 @@
 class NewGUI;
 class SGPropertyNode;
 
+/**
+ * despite looking like a subsystem, this is not, at the moment.
+ * So don't assume it has subsystem semantics or behaviour.
+ * For 'next' we're fixing this, but it's too complex a change for 2020.3
+ */
 class FGATCDialogNew
 {
 private:
@@ -52,6 +57,7 @@ public:
     void frequencyDisplay(const std::string& ident);
   
     void init();
+    void shutdown();
 
     void update(double dt);
     void PopupDialog();
@@ -63,6 +69,14 @@ public:
         return true;
     }
   
+    /**
+     * hacky fix for various crashes on reset: give a way to reset instance
+     * back to null-ptr, so that we get a new instance created. 
+     * Without this, we crash in various exciting ways due to stale pointers
+     * 
+     */
+    static void hackyReset();
+
     // Generate new instance of FGATCDialogNew if it hasn't yet been generated
 	// Call constructor and init() functions
 	// If it has been generated, will return that instance

src/Main/fg_init.cxx

@@ -1265,6 +1265,11 @@ void fgStartNewReset()
     
     subsystemManger->shutdown();
     subsystemManger->unbind();
+
+    // hack fix for many reset crashes relating to the static instance
+    // of this class. Will be fixed better for future versions by making
+    // this a proper subsystem.
+    FGATCDialogNew::hackyReset();
     
     // remove most subsystems, with a few exceptions.
     for (int g=0; g<SGSubsystemMgr::MAX_GROUPS; ++g) {