deviceEigerP11.py: add check for session being None before accessing its attributes

kamzik3/devices/deviceEigerP11.py

@@ -352,6 +352,8 @@ class EigerScanner(DeviceScanner):
         return scanner_input
 
     def _init_new_scan(self, scanner_input, scanner_attributes, parent_macro) -> None:
+        if kamzik3.session is None:
+            raise DeviceError("kamzik session is None, can't get the scan number")
         self.logger.info(
             "Initiating new scan number {}".format(
                 kamzik3.session.get_value(ATTR_SCAN_COUNT)