fix: decrease confusion for "add item" dialog when creating order

it was not clear to the user, what needs to happen when adding a new
item not yet in the system.  trick was, had to go to the Quantity tab
and choose the UOM.  hopefully this helps

src/sideshow/web/templates/orders/create.mako

@@ -801,6 +801,7 @@
                             </b-field>
 
                             <b-select v-model="productUOM"
+                                      ref="productUOM"
                                       @input="refreshTotalPrice += 1">
                               <option v-for="choice in productUnitChoices"
                                       :key="choice.key"
@@ -1232,10 +1233,6 @@
                     }
                 }
 
-                if (!this.productUOM) {
-                    return true
-                }
-
                 return false
             },
         },
@@ -1899,6 +1896,23 @@
             },
 
             itemDialogSave() {
+
+                // nb. we check this here instead of disabling the
+                // save button when no UOM is set, to avoid confusion
+                // for the user
+                if (!this.productUOM) {
+                    this.$buefy.toast.open({
+                        message: "Please specify the Unit of Measure",
+                        type: 'is-warning',
+                        duration: 2000, // 2 seconds
+                    })
+                    this.itemDialogTabIndex = 1
+                    this.$nextTick(() => {
+                        this.$refs.productUOM.focus()
+                    })
+                    return
+                }
+
                 this.itemDialogAttemptSave()
             },
         },