L2SS-667: Do not stop beam tracking if it hasn't been started

Closes L2SS-667

tangostationcontrol/tangostationcontrol/devices/beam.py

@@ -87,6 +87,13 @@ class Beam(lofar_device):
     # --------
     # overloaded functions
     # --------
+
+    def init_device(self):
+        super().init_device()
+
+        # thread to perform beam tracking
+        self.HBAT_beam_tracker = None
+
     @log_exceptions()
     def configure_for_initialise(self):
         super().configure_for_initialise()
@@ -126,8 +133,9 @@ class Beam(lofar_device):
     
     @log_exceptions()
     def configure_for_off(self):
-        # Stop thread object
-        self.HBAT_beam_tracker.stop()
+        if self.HBAT_beam_tracker:
+            # Stop thread object
+            self.HBAT_beam_tracker.stop()
 
         super().configure_for_off()