Add wrapper = null to properly destroy wrapper

As suggested by the docs

ee/spec/frontend/issuable/related_issues/components/related_issues_list_spec.js

@@ -11,7 +11,10 @@ describe('RelatedIssuesList', () => {
   let wrapper;
 
   afterEach(() => {
-    wrapper.destroy();
+    if (wrapper) {
+      wrapper.destroy();
+      wrapper = null;
+    }
   });
 
   describe('related item contents', () => {

spec/frontend/issuable/related_issues/components/add_issuable_form_spec.js

@@ -48,7 +48,10 @@ describe('AddIssuableForm', () => {
     const input = findFormInput(wrapper);
     if (input) input.blur();
 
-    wrapper.destroy();
+    if (wrapper) {
+      wrapper.destroy();
+      wrapper = null;
+    }
   });
 
   describe('with data', () => {

spec/frontend/issuable/related_issues/components/related_issues_block_spec.js

@@ -18,7 +18,10 @@ describe('RelatedIssuesBlock', () => {
   const findIssueCountBadgeAddButton = () => wrapper.find(GlButton);
 
   afterEach(() => {
-    wrapper.destroy();
+    if (wrapper) {
+      wrapper.destroy();
+      wrapper = null;
+    }
   });
 
   describe('with defaults', () => {

spec/frontend/issuable/related_issues/components/related_issues_list_spec.js

@@ -14,7 +14,10 @@ describe('RelatedIssuesList', () => {
   let wrapper;
 
   afterEach(() => {
-    wrapper.destroy();
+    if (wrapper) {
+      wrapper.destroy();
+      wrapper = null;
+    }
   });
 
   describe('with defaults', () => {