Loading stock item after creation and fixed form validation for 'dynamic' supplier part

[eeintech]

@SchrodingersGat I am not sure the behavior was intentional but after creating a stock item, there would be no redirect and the Stock index was shown and not updated. Instead I thought it would be better to show the newly created stock item, like it is showing the part details after a part creation.

Second thing, it looks like the new "dynamic" fields (#934) prevent form validation when reloaded and there is no further user interaction. Using the create stock item modal form, I was trying to select a Base part only and it would not validate the "supplier part". After immediately re-submitting, it would validate.
The way I fixed it was to remove the "supplier part" field from the form if user has not selected any item (eg. still at the default value ------). I am not sure what could be a better way, I tried to overwrite the "supplier part" field in the Form instance with a ModelChoiceField and the required=False argument but it still wouldn't validate...

[SchrodingersGat]

@eeintech oops! I've found the issue with the ------------- option - #937