Avoid having NaN in position field if you type a letter inadvertently

[vincentfretin]

From my comment in #686 (comment)
If you type inadvertently some characters for position on the right panel that are not a digit, you will get a NaN in the field.
A fix could be done to avoid the field to have NaN and put back the previous value or 0.

[vincentfretin]

I fixed it as part of #686 by putting back the previous value given in props.