[RFR][V3] TabbedFormTabs - validTabPaths fix index

[natrim]

validTabPaths needs to contain all valid elements indexed by existence
else conditional tabs leave space and are not considered valid for active tab

problem was that Children.toArray(children) strips them before map so the indexes are different from the rendering Children.map

repo: https://codesandbox.io/s/simple-i0xmz

1. edit post
2. try clicking tabs
3. you will see the index starting at 2
4. and comments not working right

[fzaninotto]

Thanks!