FIXUP: Work with terminated process group

dangerzone/isolation_provider/base.py

@@ -242,17 +242,26 @@ def terminate_doc_to_pixels_proc(
         """Terminate gracefully the process started for the doc-to-pixels phase."""
         pass
 
+    def _signal_process_group(self, p: subprocess.Popen, signo: int) -> None:
+        try:
+            os.killpg(os.getpgid(p.pid), signo)
+        except (ProcessLookupError, PermissionError):
+            # If the process no longer exists, we may encounter the above errors, either
+            # when looking for the process group (ProcessLookupError), or when trying to
+            # kill a process group that no longer exists (PermissionError)
+            return
+
     def terminate_process_group(self, p: subprocess.Popen) -> None:
         if platform.system() == "Windows":
             p.terminate()
         else:
-            os.killpg(os.getpgid(p.pid), signal.SIGTERM)
+            self._signal_process_group(signal.SIGTERM)
 
     def kill_process_group(self, p: subprocess.Popen) -> None:
         if platform.system() == "Windows":
             p.kill()
         else:
-            os.killpg(os.getpgid(p.pid), signal.SIGKILL)
+            self._signal_process_group(signal.SIGKILL)
 
     def ensure_stop_doc_to_pixels_proc(
         self,