object-manager: don't try to install if there is no core

This happens when the daemon fails to connect and the
not installed object managers try to get installed, but
the weak ref to the core is already gone

lib/wp/object-manager.c

@@ -775,11 +775,13 @@ wp_object_manager_maybe_objects_changed (WpObjectManager * self)
    */
   else if (!self->installed) {
     g_autoptr (WpCore) core = g_weak_ref_get (&self->core);
-    WpRegistry *reg = wp_core_get_registry (core);
-    if (reg->tmp_globals->len == 0 && reg->globals->len != 0) {
-      wp_trace_object (self, "installed");
-      g_signal_emit (self, signals[SIGNAL_INSTALLED], 0);
-      self->installed = TRUE;
+    if (core) {
+      WpRegistry *reg = wp_core_get_registry (core);
+      if (reg->tmp_globals->len == 0 && reg->globals->len != 0) {
+        wp_trace_object (self, "installed");
+        g_signal_emit (self, signals[SIGNAL_INSTALLED], 0);
+        self->installed = TRUE;
+      }
     }
   }
 }