Init preview pane bugfix

frontend/src/components/unification/Unification/UnificationRightPane.vue

@@ -18,7 +18,7 @@ export default {
         key: this.lexicalUnitId,
         entryIdLocal: this.entryId,
         unifiedFrameId: this.initialUnifiedFrameId,
-        previewedUnifiedFrameId: null
+        previewedUnifiedFrameId: -1
       };
     },
     goToDisplay() {

frontend/src/components/unification/hierarchy/HierarchyRightPane.vue

@@ -18,7 +18,7 @@ export default {
         key: this.lexicalUnitId,
         entryIdLocal: this.entryId,
         unifiedFrameId: this.initialUnifiedFrameId,
-        previewedUnifiedFrameId: null
+        previewedUnifiedFrameId: -1
       };
     },
     goToDisplay() {