fix: use safe navigation for model.context.label in form template

model.context might be undefined for default DSpace form array models
that don't set context.label. Use optional chaining (?.) to prevent
runtime crash, with fallback to form.add translation.

Author: milanmajchrak

src/app/shared/form/form.component.html

@@ -27,10 +27,10 @@
         <div *ngIf="(!context.notRepeatable) && index === (group.context.groups.length - 1) && !isItemReadOnly(context, index)" class="clearfix pl-4 w-100">
           <div class="btn-group" role="group">
             <button type="button" role="button" class="ds-form-add-more btn btn-link"
-                    title="{{'form.add' | translate}} {{ model.context.label }}"
-                    attr.aria-label="{{'form.add' | translate}} {{ model.context.label }}"
+                    title="{{'form.add' | translate}} {{ model?.context?.label }}"
+                    attr.aria-label="{{'form.add' | translate}} {{ model?.context?.label }}"
                     (click)="insertItem($event, group.context, group.context.groups.length)">
-              <span><i class="fas fa-plus"></i>&nbsp;{{ model.context.label }}</span>
+              <span><i class="fas fa-plus"></i>&nbsp;{{ model?.context?.label || ('form.add' | translate) }}</span>
               <!-- DATASHARE: Replaced {{'form.add' | translate}} with {{ model.context.label }} -->
             </button>
           </div>