Improve the addin

[haozhu233]

Description

• show empty table instead of error when search returns nothing;
• the action button on the DOI panel now also changes to "added" after clicked

[codecov-io]

Codecov Report

Merging #153 into master will decrease coverage by 0.21%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   52.25%   52.04%   -0.22%     
==========================================
  Files          28       28              
  Lines        1483     1489       +6     
==========================================
  Hits          775      775              
- Misses        708      714       +6

Impacted Files	Coverage Δ
R/crAddins.R	0% <0%> (ø)	:arrow_up:

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