Check waitable for nullptr during constructor

Signed-off-by: Stephen Brawner <brawner@gmail.com>

rclcpp/include/rclcpp/wait_set_policies/detail/storage_policy_common.hpp

@@ -73,7 +73,17 @@ class StoragePolicyCommon
     size_t services_from_waitables = 0;
     size_t events_from_waitables = 0;
     for (const auto & waitable_entry : waitables) {
-      rclcpp::Waitable & waitable = *waitable_entry.waitable.get();
+      auto waitable_ptr_pair = get_raw_pointer_from_smart_pointer(waitable_entry.waitable);
+      if (nullptr == waitable_ptr_pair.second) {
+        if (HasStrongOwnership) {
+          throw std::runtime_error("unexpected condition, fixed storage policy needs pruning");
+        }
+        // Flag for pruning.
+        needs_pruning_ = true;
+        continue;
+      }
+
+      rclcpp::Waitable & waitable = *waitable_ptr_pair.second;
       subscriptions_from_waitables += waitable.get_number_of_ready_subscriptions();
       guard_conditions_from_waitables += waitable.get_number_of_ready_guard_conditions();
       timers_from_waitables += waitable.get_number_of_ready_timers();