Fix bug closing Tabs

[philippjfr]

Fixes #1188

[codecov]

Codecov Report

Merging #1208 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1208      +/-   ##
==========================================
- Coverage   87.63%   87.62%   -0.01%     
==========================================
  Files         106      106              
  Lines       12588    12709     +121     
==========================================
+ Hits        11031    11136     +105     
- Misses       1557     1573      +16     

Impacted Files	Coverage Δ
panel/layout.py	84.76% <100.00%> (+0.06%)	⬆️
panel/io/save.py	78.37% <0.00%> (-4.06%)	⬇️
panel/io/state.py	89.70% <0.00%> (-0.50%)	⬇️
panel/util.py	88.05% <0.00%> (+2.86%)	⬆️

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