Field height jumps when errors appear because of `helperText`

[Philipp91]

A field without error and warning has no helperText set. In that case, material-ui's <TextField/> will not reserve any space for the helper text. When the field becomes validated and is invalid, the errorText is added and the height of the field increases. This leads to a visible jump in the page layout, which is especially bad if there are many invalid fields, or if it happens often as users click through the fields.

Any proper solution will have to reserve this space always, even when no error message is present. This leads to a less dense form. This was the default behavior with redux-form-material-ui 4.x and material-ui 0.x, but it broke with the new versions.

[arno-le]

Is this going to be merged?