prior: fix AttributeError for bad connections

When constructing a ProScanIII object on a port that does
not contain a prior controller, the _devices member is not
defined.

This causes the __del__ operation of the object to fail, because
it will end up in the base class' __del__, which requires the
`devices` property, which requires the `_devices` member to be present.

Conceptually, the base class requires the implementation to be
a valid object.  One may also argue that the object is not allowed
to be construted at all if not connected to a proper device; that
would be an alternative fix.

Author: Kristoffel Pirard

microscope/controllers/prior.py

@@ -211,15 +211,17 @@ class ProScanIII(microscope.abc.Controller):
     def __init__(
         self, port: str, baudrate: int = 9600, timeout: float = 0.5, **kwargs
     ) -> None:
-        super().__init__(**kwargs)
-        self._conn = _ProScanIIIConnection(port, baudrate, timeout)
         self._devices: Mapping[str, microscope.abc.Device] = {}
 
-        # Can have up to three filter wheels, numbered 1 to 3.
-        for number in range(1, 4):
-            if self._conn.has_filterwheel(number):
-                key = "filter %d" % number
-                self._devices[key] = _ProScanIIIFilterWheel(self._conn, number)
+        self._conn = _ProScanIIIConnection(port, baudrate, timeout)
+
+        self._devices = {
+            "filter %d" % number: _ProScanIIIFilterWheel(self._conn, number)
+            for number in range(1, 4)  # Can have up to three filter wheels, numbered 1 to 3.
+            if self._conn.has_filterwheel(number)
+        }
+
+        super().__init__(**kwargs)
 
     @property
     def devices(self) -> Mapping[str, microscope.abc.Device]: