more protection

owrx/source.py

@@ -88,6 +88,7 @@ class SdrSource(object):
         self.clients = []
         self.spectrumClients = []
         self.spectrumThread = None
+        self.process = None
         self.modificationLock = threading.Lock()
 
         # override these in subclasses as necessary
@@ -184,8 +185,10 @@ class SdrSource(object):
 
         self.modificationLock.acquire()
 
+        if self.spectrumThread is not None:
             self.spectrumThread.stop()
 
+        if self.process is not None:
             try:
                 os.killpg(os.getpgid(self.process.pid), signal.SIGTERM)
             except ProcessLookupError: