Ensure non-existent models in Template do not cause errors

[philippjfr]

Certain components do not render their models immediately instead preferring to do so lazily. The Template however expects all models to be available on initial render. This PR ensures that components that have not yet been instantiated are simply skipped avoiding any resultant errors.

[codecov]

Codecov Report

Merging #1374 into master will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1374      +/-   ##
==========================================
- Coverage   87.69%   87.68%   -0.01%     
==========================================
  Files         130      130              
  Lines       13922    13927       +5     
==========================================
+ Hits        12209    12212       +3     
- Misses       1713     1715       +2     

Impacted Files	Coverage Δ
panel/template/base.py	56.25% <66.66%> (+0.08%)	⬆️

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[MarcSkovMadsen]

Does that mean you can have a template with {{ panel_10 }} but not add_panel with name panel_10? That would be nice.

[philippjfr]

No, totally unrelated to that but please open an issue about that.