fix: fix types for TypeScript 3.8

[dangreen]

No description provided.

[codecov-commenter]

Codecov Report

Merging #420 (da0c9e4) into master (1afd218) will increase coverage by 0.02%.
The diff coverage is 43.54%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   42.27%   42.29%   +0.02%     
==========================================
  Files          16       15       -1     
  Lines         537      532       -5     
  Branches      112      118       +6     
==========================================
- Hits          227      225       -2     
+ Misses        250      246       -4     
- Partials       60       61       +1     

Impacted Files	Coverage Δ
src/components/GoogleChartActions.tsx	0.00% <0.00%> (ø)
src/default-props.ts	100.00% <ø> (ø)
src/types.ts	0.00% <ø> (ø)
src/Context.tsx	70.83% <33.33%> (ø)
src/ReactGoogleCharts.tsx	55.35% <37.50%> (ø)
src/components/GoogleChart.tsx	42.01% <45.16%> (-1.58%)	⬇️
src/components/GoogleChartDataTable.tsx	43.62% <50.00%> (-0.68%)	⬇️
src/components/GoogleChartEvents.tsx	51.72% <50.00%> (ø)
... and 5 more

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