Merge branch 'fix-corpus-id-undefined' into 'master'

Prevent TypeErrors when looking for an element's corpusId

Closes #1004

See merge request !1286

vue/Element/Main.vue

@@ -144,10 +144,14 @@ export default {
     element () {
       return this.elements[this.id]
     },
+    /**
+     * The corporaMixin requires a corpusId property to be defined to be able to retrieve the current corpus.
+     * It should either return a corpus ID, or null is there is no corpus.
+     * Returning `undefined` can cause a warning in the console, as the mixin thinks the property may not be defined.
+     * @returns {string | null}
+     */
     corpusId () {
-      // Corpus ID for the corpora mixin
-      if (!this.element) return null
-      return this.element.corpus.id
+      return this.element?.corpus?.id ?? null
     },
     elementType () {
       return this.element && this.getType(this.element.type)