[BUG] Handle missing `timing_information` gracefully

[remibaar]

Describe your context

dash                      1.17.0                  
dash-core-components      1.13.0             
dash-html-components      1.1.1              
dash-renderer             1.8.3              

Describe the bug
An error is raised when the devtools before request is not fully executed.
AttributeError: '_AppCtxGlobals' object has no attribute 'timing_information'
In our case this happens because we register our own "before request" to the flask instance, to handle user logins. Flask will not execute any other before_request_funcs when a function returns something other than None. If an user is not logged in, we will throw a redirect to the login page.

So during the "before_request" phase the timing_information is not set. During the "after_request" phase, it expects timing_information was set. If not it will throw an error.

The error is caused by these lines: https://github.com/plotly/dash/blob/master/dash/dash.py#L1465-L1489

Expected behavior
It should check if the timing_information is set, and not expect it is. If it is not it gracefully should handle the error.

[anders-kiaer]

Similar to #1475?

[remibaar]

Indeed it is a duplicate of #1475
I will close this issue, and create a PR to fix it