[client] Several ts fixes.

This MR fixes more or les 30 tsc errors by :

• migrating 2 files to typescript (they use React.forwardRef which creates typing interferences issues)
• Ensure backend promisses always retrun a promise
• Initialize useRef hook with null as refs are expected to be either a node or null (see: https://linguinecode.com/post/how-to-use-react-useref-with-typescript#useref-examples , you can find this implementation on the official doc: https://reactjs.org/docs/hooks-reference.html#useref
• Prefix name of unused parameters with _ (see here: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html#example-18)