null-check the entry in ImportBlackboardFromJSON after createEntry (#1284)

* null-check the entry in ImportBlackboardFromJSON after createEntry

After createEntry() the entry was re-fetched with getEntry() and dereferenced
unchecked; a concurrent Blackboard::unset() from another thread in that window
returned nullptr and crashed. Skip that key instead.

No test: the window is a few instructions wide, so a deterministic reproduction
is not possible. The same createEntry->getEntry->deref pattern elsewhere
(SetBlackboardNode, script assignment operator) is already null-checked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* reduce nesting of the null check (Sonar S134)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: facontidavide

src/blackboard.cpp

@@ -536,6 +536,10 @@ void ImportBlackboardFromJSON(const nlohmann::json& json, Blackboard& blackboard
         blackboard.createEntry(it.key(), res->second);
         entry = blackboard.getEntry(it.key());
       }
+      if(!entry)
+      {
+        continue;  // a concurrent unset() removed it right after createEntry()
+      }
       // Lock entry_mutex before writing to prevent data races (BUG-4 fix).
       std::shared_ptr<Blackboard::EntryUpdatedSignal> update_signal;
       {