Try a different way of fixing userscript crashes

CHANGELOG.asciidoc

@@ -130,6 +130,7 @@ Fixed
 - (QtWebEngine) HTML fullscreen is now tracked for each tab separately, which
   means it's not possible anymore to accidentally get stuck in fullscreen state
   by closing a tab with a fullscreen video.
+- For some people, running some userscripts crashed - this should now be fixed.
 - Various other rare crashes should now be fixed.
 - The settings documentation was truncated with v0.10.1 which should now be
   fixed.

qutebrowser/commands/userscripts.py

@@ -64,14 +64,19 @@ class _QtFIFOReader(QObject):
     def read_line(self):
         """(Try to) read a line from the FIFO."""
         log.procs.debug("QSocketNotifier triggered!")
-        self._notifier.setEnabled(False)
         try:
-            for line in self._fifo:
-                self.got_line.emit(line.rstrip('\r\n'))
-                self._notifier.setEnabled(True)
-        except UnicodeDecodeError as e:
-            log.misc.error("Invalid unicode in userscript output: {}"
-                           .format(e))
+            self._notifier.setEnabled(False)
+            try:
+                for line in self._fifo:
+                    self.got_line.emit(line.rstrip('\r\n'))
+                    self._notifier.setEnabled(True)
+            except UnicodeDecodeError as e:
+                log.misc.error("Invalid unicode in userscript output: {}"
+                               .format(e))
+        except RuntimeError as e:
+            # For unknown reasons, read_line can still get called after the
+            # QSocketNotifier was already deleted...
+            log.procs.debug("While reading userscript output: {}".format(e))
 
     def cleanup(self):
         """Clean up so the FIFO can be closed."""