Prevented creating picking list with no items.

prs.desktop/Panels/Manufacturing/ManufacturingPanelColumn.xaml.cs

@@ -751,7 +751,8 @@ namespace PRSDesktop
 
             var components =
                 Client.Query(
-                    new Filter<ManufacturingPacketComponent>(x => x.Packet.ID).InList(pkts.Select(x => x.ID).ToArray()),
+                    new Filter<ManufacturingPacketComponent>(x => x.Packet.ID).InList(pkts.Select(x => x.ID).ToArray())
+                        .And(x => x.Requisition.ID).IsEqualTo(Guid.Empty),
                     new Columns<ManufacturingPacketComponent>()
                         .Add(x => x.ID)
                         .Add(x => x.Description)
@@ -796,6 +797,11 @@ namespace PRSDesktop
 
                 requisitionItems.Add(requisitionItem);
             }
+            if(requisitionItems.Count == 0)
+            {
+                MessageBox.Show("No items to create picking list; all selected components already have requisitions.");
+                return;
+            }
 
             var requisition = new Requisition
             {