Fix unhandled SelectionUnsupportedError in :completion-item-yank

doc/changelog.asciidoc

@@ -67,6 +67,8 @@ Fixed
 
 - Crash when `tabs.focus_stack_size` is set to -1.
 - Crash when a `pdf.js` file for PDF.js exists, but `viewer.html` does not.
+- Crash when `:completion-item-yank --sel` is used on a platform without
+  primary selection support (e.g. Windows/macOS).
 - `:config-write-py` now works with paths starting with `~/...` again.
 - New site-specific quirk for a missing `globalThis` in Qt <= 5.12 on Reddit and Spotify
 

qutebrowser/completion/completionwidget.py

@@ -424,4 +424,8 @@ class CompletionView(QTreeView):
             if not index.isValid():
                 raise cmdutils.CommandError("No item selected!")
             text = self.model().data(index)
+
+        if not utils.supports_selection():
+            sel = False
+
         utils.set_clipboard(text, selection=sel)