[FIX] purchase: hide send reminder on unsaved record

Before this commit, Button to Send Sample reminder was always visible which doesn't makes sense as It would work on saved record only.

Now Button will be hidden on unsaved record.

Fixes #58744

closes odoo/odoo#59799

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>

addons/purchase/views/purchase_views.xml

@@ -175,6 +175,7 @@
                             />
                             <field name="partner_ref"/>
                             <field name="currency_id" groups="base.group_multi_currency" force_save="1"/>
+                            <field name="id" invisible="1"/>
                         </group>
                         <group>
                             <field name="date_order" attrs="{'invisible': [('state','=','purchase')]}"/>
@@ -197,7 +198,7 @@
                                 <div class="o_row oe_inline" attrs="{'invisible': [('receipt_reminder_email', '=', False)]}">
                                     <field name="reminder_date_before_receipt" class="oe_inline"/>
                                     days before 
-                                    <widget name='toaster_button' button_name="send_reminder_preview" title="Preview the reminder email by sending it to yourself."/>
+                                    <widget name='toaster_button' button_name="send_reminder_preview" title="Preview the reminder email by sending it to yourself." attrs="{'invisible': [('id', '=', False)]}"/>
                                 </div>
                             </div>
                             <field name="origin" attrs="{'invisible': [('origin','=',False)]}"/>