Initialize with submitted value on_hand

app/controllers/spree/admin/products_controller.rb

@@ -36,6 +36,8 @@ module Spree
               redirect_to spree.admin_products_path
             end
           else
+            # Re-fill the form with deleted params on product
+            @on_hand = request.params[:product][:on_hand]
             render :new
           end
         end

app/views/spree/admin/products/new.html.haml

@@ -77,7 +77,7 @@
           = f.field_container :on_hand do
             = f.label :on_hand, t(".on_hand")
             %br/
-            = f.text_field :on_hand, class: 'fullwidth'
+            = f.text_field :on_hand, class: 'fullwidth', value: @on_hand
             = f.error_message_on :on_hand
         .three.columns.omega
           = f.field_container :on_demand do