Fix adding and deleting Location component

[philippjfr]

The Location component lookup was broken in certain scenarios causing errors.

[codecov]

Codecov Report

Merging #1519 into master will decrease coverage by 0.03%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master    #1519      +/-   ##
==========================================
- Coverage   85.66%   85.62%   -0.04%     
==========================================
  Files         144      144              
  Lines       15951    15959       +8     
==========================================
+ Hits        13664    13665       +1     
- Misses       2287     2294       +7     

Impacted Files	Coverage Δ
panel/viewable.py	70.79% <0.00%> (ø)
panel/io/location.py	80.89% <12.50%> (-6.76%)	⬇️

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