[DataGrid] getValueError in valueGetter if incorrect field is supplied

[ZeeshanTamboli]

Fixes #749

Render empty row data if column field for valueGetter method is incorrect instead of app crashing.

Not sure if throwing error in console instead would be a better idea like:

 const col = api.getColumnFromField(field);
 if(!col) {
    throw new Error(`Material-UI: The field ${field} supplied to valueGetter element is invalid.`);
 }
 if (!col.valueGetter) {
   return rowModel[field];
 }

[oliviertassinari]

I was wondering too about what should be the expected outcome of getValue for a column that isn't defined. I have pushed into the direction of gracefully working, for the sake of simplicity. getValue exists in the first place to call valueGetter if it exists. I guess that some developers might want to always use it. At least, the ones starting from the demos of the documentation. But we could also say the opposite, go pull data from the raw data instead. No matter what, we need to change something, we can't fail with an obscure error.

[oliviertassinari]

@ZeeshanTamboli Thanks for spending time on this! I hope the final solution matches what you had expected. Happy to continue the conversion at any point. In #755 (comment), we still need to test that the returned value is correct as the branching could have been implemented in a different way, for instead, before the changes, it was crashing.

[ZeeshanTamboli]

@ZeeshanTamboli Thanks for spending time on this! I hope the final solution matches what you had expected. Happy to continue the conversion at any point. In #755 (comment), we still need to test that the returned value is correct as the branching could have been implemented in a different way, for instead, before the changes, it was crashing.

@oliviertassinari Thanks! Happy to help! Saw the implementation now, and it's even better to show the warning. Looking forward to contribute more hopefully. :)