[OMCSessionZMQ] verify that _omc is not None in sendExpression()

Author: syntron

OMPython/OMCSession.py

@@ -526,6 +526,9 @@ def sendExpression(self, command, parsed=True):
         if p is not None:
             raise OMCSessionException("Process Exited, No connection with OMC. Create a new instance of OMCSessionZMQ!")
 
+        if self._omc is None:
+            raise OMCSessionException("No OMC running. Create a new instance of OMCSessionZMQ!")
+
         attempts = 0
         while True:
             try: