Update theta_star_planner.cpp (ros-navigation#3918)

Author: SteveMacenski

nav2_theta_star_planner/src/theta_star_planner.cpp

@@ -91,6 +91,8 @@ nav_msgs::msg::Path ThetaStarPlanner::createPlan(
   nav_msgs::msg::Path global_path;
   auto start_time = std::chrono::steady_clock::now();
 
+  std::unique_lock<nav2_costmap_2d::Costmap2D::mutex_t> lock(*(planner_->costmap_->getMutex()));
+
   // Corner case of start and goal beeing on the same cell
   unsigned int mx_start, my_start, mx_goal, my_goal;
   if (!planner_->costmap_->worldToMap(