Do not declare variable until necessary

src/PIL/ImageGrab.py

@@ -157,14 +157,14 @@ def grabclipboard():
             raise NotImplementedError(msg)
 
         p = subprocess.run(args, capture_output=True)
-        err = p.stderr
         if p.returncode != 0:
             allowed_errors = [
                 b"Nothing is copied",  # wl-paste, when the clipboard is empty
                 b"not available",  # wl-paste/debian xclip, when an image isn't available
                 b"cannot convert",  # xclip, when an image isn't available
                 b"There is no owner",  # xclip, when the clipboard isn't initialized
             ]
+            err = p.stderr
             if any(e in err for e in allowed_errors):
                 return None
             msg = f"{args[0]} error: {err.strip().decode() if err else 'Unknown error'}"