fix: kinematicunit might not be available on startup and causes a nullptr exception.

source/RobotAPI/libraries/armem_robot_state/server/description/Segment.cpp

 #include "Segment.h"
 
+#include <IceUtil/Time.h>
 #include <RobotAPI/libraries/armem_robot/aron/RobotDescription.aron.generated.h>
 #include <sstream>
 
+#include "ArmarXCore/core/exceptions/local/ExpressionException.h"
 #include "ArmarXCore/core/logging/Logging.h"
 #include <ArmarXCore/core/time/TimeUtil.h>
 
@@ -20,6 +22,7 @@
 #include <RobotAPI/libraries/armem_robot/aron/Robot.aron.generated.h>
 #include <RobotAPI/libraries/armem_robot/aron_conversions.h>
 #include <RobotAPI/libraries/armem_robot/robot_conversions.h>
+#include <thread>
 
 namespace armarx::armem::server::robot_state::description
 {
@@ -93,7 +96,27 @@ namespace armarx::armem::server::robot_state::description
 
     void Segment::updateRobotDescription()
     {
-        auto kinematicUnit = robotUnit->getKinematicUnit();
+        ARMARX_CHECK_NOT_NULL(robotUnit);
+
+        const auto waitForKinematicUnit = [&]()
+        {
+            while (true)
+            {
+                auto kinematicUnit = robotUnit->getKinematicUnit();
+
+                if (kinematicUnit)
+                {
+                    ARMARX_INFO << "Kinematic unit is now available.";
+                    return kinematicUnit;
+                }
+
+                ARMARX_INFO << "Waiting for kinematic unit ...";
+                std::this_thread::sleep_for(std::chrono::seconds(1));
+            }
+        };
+
+        auto kinematicUnit = waitForKinematicUnit();
+        ARMARX_CHECK_NOT_NULL(kinematicUnit);
 
         const auto robotName = kinematicUnit->getRobotName();