ipc: Initialize debug GUI after system

If we initialize the debug GUI first, then it will attempt to access a NULL system and fail.

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2638>

src/xrt/ipc/server/ipc_server_process.c

···
 		return -1;
 	}
 
-	// Start the debug UI now (if enabled).
-	u_debug_gui_start(s->debug_gui, s->xinst, s->xsysd);
-
 	// Tell the callbacks we are entering the main-loop.
 	callbacks->mainloop_entering(s, s->xinst, data);
 
···
 	if (xret != XRT_SUCCESS) {
 		U_LOG_CHK_ONLY_PRINT(log_level, xret, "ipc_server_init_system_if_available_locked");
 	}
+
+	// Start the debug UI now (if enabled).
+	u_debug_gui_start(s->debug_gui, s->xinst, s->xsysd);
 
 	// Main loop.
 	ret = main_loop(s);