Check if dbus handler is null before using

src/main.rs

@@ -159,8 +159,10 @@ mod c {
         };
 
         if let Some(visible) = msg.dbus_visible_set {
+            if dbus_handler != std::ptr::null() {
                 unsafe { dbus_handler_set_visible(dbus_handler, visible as u8) };
             }
+        }
 
         if let Some(hints) = msg.layout_hint_set {
             unsafe {