Fix for Pack operator's poll erro (#119)

There was an error visible in console, when user removed export node, looks like it tried to poll the last item in the list that didn't exist anymore

albam/blender_ui/export_panel.py

@@ -275,12 +275,19 @@ def _execute(self, context):  # pragma: no cover
     def poll(cls, context):
         vfs_e = context.scene.albam.exported
         vfs_i = context.scene.albam.vfs
+        # TODO: simplify this mess
         if len(vfs_e.file_list) == 0 or len(vfs_i.file_list) == 0:
             return False
         index_e = vfs_e.file_list_selected_index
         index_i = vfs_i.file_list_selected_index
-        exported_item = vfs_e.file_list[index_e]
-        imported_item = vfs_i.file_list[index_i]
+        if index_e < (len(vfs_e.file_list)):
+            exported_item = vfs_e.file_list[index_e]
+        else:
+            return False
+        if index_i < (len(vfs_i.file_list)):
+            imported_item = vfs_i.file_list[index_i]
+        else:
+            return False
         if not exported_item or not imported_item:
             return False
         return imported_item.is_archive and exported_item.is_root