Fix for loading parameter widget linking

[philippjfr]

Fixes #2158

[codecov]

Codecov Report

Merging #2160 (7e85f84) into master (6d36858) will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2160      +/-   ##
==========================================
- Coverage   83.78%   83.75%   -0.03%     
==========================================
  Files         182      182              
  Lines       21679    21680       +1     
==========================================
- Hits        18164    18159       -5     
- Misses       3515     3521       +6     

Impacted Files	Coverage Δ
panel/param.py	87.92% <50.00%> (+0.02%)	⬆️
panel/tests/pane/test_base.py	88.23% <0.00%> (-11.77%)	⬇️
panel/pane/base.py	87.89% <0.00%> (-0.90%)	⬇️

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