frontend-tools: Fix crash on non X11 windowing systems

(cherry picked from commit 7d26721be59d7818721f9f7a2ababb49521e6b15)

UI/frontend-plugins/frontend-tools/auto-scene-switcher-nix.cpp

@@ -46,8 +46,14 @@ static bool ewmhIsSupported()
 	unsigned long num = 0, bytes = 0;
 	unsigned char *data = NULL;
 	Window ewmh_window = 0;
+	Window root_window = 0;
 
-	int status = XGetWindowProperty(display, DefaultRootWindow(display),
+	root_window = DefaultRootWindow(display);
+	if (!root_window) {
+		return false;
+	}
+
+	int status = XGetWindowProperty(display, root_window,
 					netSupportingWmCheck, 0L, 1L, false,
 					XA_WINDOW, &actualType, &format, &num,
 					&bytes, &data);
@@ -97,6 +103,9 @@ static std::vector<Window> getTopLevelWindows()
 
 	for (int i = 0; i < ScreenCount(disp()); ++i) {
 		Window rootWin = RootWindow(disp(), i);
+		if (!rootWin) {
+			continue;
+		}
 
 		int status = XGetWindowProperty(disp(), rootWin, netClList, 0L,
 						~0L, false, AnyPropertyType,
@@ -119,6 +128,9 @@ static std::vector<Window> getTopLevelWindows()
 static std::string GetWindowTitle(size_t i)
 {
 	Window w = getTopLevelWindows().at(i);
+	if (!w) {
+		return "";
+	}
 	std::string windowTitle;
 	char *name;
 
@@ -164,11 +176,17 @@ void GetCurrentWindowTitle(string &title)
 	char *name;
 
 	Window rootWin = RootWindow(disp(), 0);
+	if (!rootWin) {
+		return;
+	}
 
 	XGetWindowProperty(disp(), rootWin, active, 0L, ~0L, false,
 			   AnyPropertyType, &actualType, &format, &num, &bytes,
 			   (uint8_t **)&data);
 
+	if (!data[0]) {
+		return;
+	}
 	int status = XFetchName(disp(), data[0], &name);
 
 	if (status >= Success && name != nullptr) {