fix jalhyd#106 PAB deserialisation

src/app/services/formulaire/formulaire.service.ts

@@ -334,8 +334,13 @@ export class FormulaireService extends Observable {
             }
 
             // when creating a new Pab, add one wall with one device, plus the downwall
-            // (when loading session only)
-            if (f instanceof FormulairePab && f.currentNub instanceof Pab) {
+            // (when loading session, those items are already present)
+            if (
+                f instanceof FormulairePab
+                && f.currentNub instanceof Pab
+                && f.currentNub.children.length === 0
+                && f.currentNub.downWall === undefined
+            ) {
                 // 1. one wall
                 const newWall = Session.getInstance().createNub(
                     new Props({