fix: FAB_INDEX_VIEW type check

[dpgaspar]

Description

fixes FAB_INDEX_VIEW that was failing with

Traceback (most recent call last):
  File "/app/superset/app.py", line 37, in create_app
    app_initializer.init_app()
  File "/app/superset/initialization/__init__.py", line 460, in init_app
    self.init_app_in_ctx()
  File "/app/superset/initialization/__init__.py", line 408, in init_app_in_ctx
    self.configure_fab()
  File "/app/superset/initialization/__init__.py", line 500, in configure_fab
    appbuilder.init_app(self.superset_app, db.session)
  File "/usr/local/lib/python3.8/site-packages/flask_appbuilder/base.py", line 177, in init_app
    if isinstance(view, BaseView):
NameError: name 'BaseView' is not defined

Also fixes type annotations on AppBuilder

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1883 (57cb59b) into master (449afe4) will decrease coverage by 0.27%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master    #1883      +/-   ##
==========================================
- Coverage   77.64%   77.37%   -0.28%     
==========================================
  Files          70       70              
  Lines        8559     8420     -139     
==========================================
- Hits         6646     6515     -131     
+ Misses       1913     1905       -8     

Flag	Coverage Δ
python	77.37% <86.66%> (-0.28%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask_appbuilder/base.py	78.85% <86.66%> (+0.79%)	⬆️
flask_appbuilder/models/mixins.py	69.23% <0.00%> (-4.11%)	⬇️
flask_appbuilder/cli.py	55.98% <0.00%> (-2.95%)	⬇️
flask_appbuilder/views.py	63.85% <0.00%> (-2.26%)	⬇️
flask_appbuilder/filters.py	89.79% <0.00%> (-1.20%)	⬇️
flask_appbuilder/security/manager.py	74.45% <0.00%> (-0.68%)	⬇️
flask_appbuilder/babel/views.py	92.30% <0.00%> (-0.55%)	⬇️
flask_appbuilder/security/registerviews.py	31.38% <0.00%> (-0.50%)	⬇️
flask_appbuilder/models/group.py	72.31% <0.00%> (-0.47%)	⬇️
flask_appbuilder/api/manager.py	92.00% <0.00%> (-0.46%)	⬇️
... and 16 more

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