Centralize handling of modal error display.

firmware/application/analog_audio_app.cpp

@@ -140,7 +140,7 @@ AnalogAudioView::AnalogAudioView(
 	};
 
 	record_view.on_error = [&nav](std::string message) {
-		nav.push<ModalMessageView>(message);
+		nav.display_error(message);
 	};
 
 	audio::output::start();

firmware/application/capture_app.cpp

@@ -71,7 +71,7 @@ CaptureAppView::CaptureAppView(NavigationView& nav) {
 
 	record_view.set_sampling_rate(sampling_rate / 8);
 	record_view.on_error = [&nav](std::string message) {
-		nav.push<ModalMessageView>(message);
+		nav.display_error(message);
 	};
 }
 

firmware/application/ui_navigation.cpp

@@ -117,6 +117,10 @@ View* NavigationView::push_view(std::unique_ptr<View> new_view) {
 }
 
 void NavigationView::pop() {
+	if( view() == modal_view ) {
+		modal_view = nullptr;
+	}
+
 	// Can't pop last item from stack.
 	if( view_stack.size() > 1 ) {
 		free_view();
@@ -127,6 +131,13 @@ void NavigationView::pop() {
 	}
 }
 
+void NavigationView::display_error(const std::string& message) {
+	/* If a modal view is already visible, don't display another */
+	if( !modal_view ) {
+		modal_view = push<ModalMessageView>(message);
+	}
+}
+
 void NavigationView::free_view() {
 	remove_child(view());
 }

firmware/application/ui_navigation.hpp

@@ -102,10 +102,13 @@ public:
 
 	void pop();
 
+	void display_error(const std::string& message);
+
 	void focus() override;
 
 private:
 	std::vector<std::unique_ptr<View>> view_stack;
+	Widget* modal_view { nullptr };
 
 	Widget* view() const;