[OMCSessionZMQ] verify that _omc is not None before trying to close it

Author: syntron

OMPython/OMCSession.py

@@ -541,8 +541,9 @@ def sendExpression(self, command, parsed=True):
                 raise OMCSessionException(f"No connection with OMC (timeout={self._timeout}). Log-file says: \n{log}")
             time.sleep(self._timeout / 50.0)
         if command == "quit()":
-            self._omc.close()
-            self._omc = None
+            if self._omc is not None:
+                self._omc.close()
+                self._omc = None
             return None
         else:
             result = self._omc.recv_string()