Theme callback called twice

[oliviertassinari]

This issue is part of a series of feedbacks. I been upgrading the Material-UI solution.
Unfortunately, I had to fork the package in order to reach my goal.
Now, it's time to reconciliation.

The issue

I have noticed that a theme callback can be called twice during a first render.
The first time, it's called with a null outer theme.
I think that it can be reproduced with the following:

<ThemeProvider theme={x}>
  <ThemeProvider theme{(x) => { console.log(x) }>
    <div />
  </ThemeProvider>
</ThemeProvider>