Fix again

Change-Id: I7f2052a19afa72e1df8e6bbe9305c752d69e21a9

entangle/device/thermofisher.py

@@ -200,6 +200,9 @@ class XRaySource(base.DiscreteOutput):
         return self._state
 
     def write_value(self, value):
-        value = {1: 'ON', 0: 'OFF'}.value
-        self._iodev.Communicate(f'XRAY {value}')
+        value = {1: 'ON', 0: 'OFF'}.get(value, None)
+        if value:
+            self._iodev.Communicate(f'XRAY {value}')
+        else:
+            raise InvalidValue(f"Can't write {value} as target for this device.")
         self.state()
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