Fix file redirect error

voila/handler.py

@@ -127,7 +127,10 @@ async def get_generator(self, path=None):
                 kernel_spec_manager=self.kernel_spec_manager,
             )
 
-            await gen.initialize(template=template_arg, theme=theme_arg)
+            done = await gen.initialize(template=template_arg, theme=theme_arg)
+            if not done:
+                self.redirect_to_file(path)
+                return
 
             def time_out():
                 """If not done within the timeout, we send a heartbeat

voila/notebook_renderer.py

@@ -12,7 +12,6 @@
 import sys
 import traceback
 from typing import Generator, Tuple, Union, List
-
 import nbformat
 import tornado.web
 from jupyter_server.config_manager import recursive_update
@@ -46,8 +45,14 @@ def __init__(self, **kwargs):
         self.stop_generator = False
         self.rendered_cache: List[str] = []
 
-    async def initialize(self, **kwargs):
+    async def initialize(self, **kwargs) -> bool:
+        """ Initialize the notebook generator, this method will
+        return `True` if the generator is initialized, return `False`
+        otherwise.
 
+        Returns:
+            Boolean: The flag to check if the generator is initialized.
+        """
         notebook_path = self.notebook_path
         if self.voila_configuration.enable_nbextensions:
             # generate a list of nbextensions that are enabled for the classical notebook
@@ -68,7 +73,7 @@ async def initialize(self, **kwargs):
         self.notebook = await self.load_notebook(notebook_path)
 
         if not self.notebook:
-            return
+            return False
         self.cwd = os.path.dirname(notebook_path)
 
         _, basename = os.path.split(notebook_path)
@@ -145,6 +150,8 @@ async def initialize(self, **kwargs):
             self.exporter.exclude_output_prompt = True
             self.exporter.exclude_input_prompt = True
 
+        return True
+
     def generate_content_generator(
         self,
         kernel_id: Union[str, None] = None,