Fix missing nbformat version

Author: fcollonval

jupyter_ydoc/ynotebook.py

@@ -202,7 +202,7 @@ def get(self) -> Dict:
         cells = []
         for i in range(len(self._ycells)):
             cell = self.get_cell(i)
-            if "id" in cell and meta["nbformat"] == 4 and meta["nbformat_minor"] <= 4:
+            if "id" in cell and meta.get("nbformat", 0) == 4 and meta.get("nbformat_minor", 0) <= 4:
                 # strip cell IDs if we have notebook format 4.0-4.4
                 del cell["id"]
             if (