form: combo grid: use correct method to initialize the picker

form: combo grid: use correct method to initialize the picker

'createPicker' does create the picker, but saves no reference to it
in the owning field, Instead, 'getPicker' should be used for that,
since that does all the necessary initialization, and actually
assigns the created picker to the field's me.picker variable so that
the pickers lifetime can be successfully tracked and enforced.

Without this patch, we leak the picker being created every time a
combo grid is created.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>