Fix a couple of possibly-null errors from TS

Add checks or tweak the code to make it obvious to TS that a value is
not-null.

src/sidebar/components/group-list-item.js

@@ -82,9 +82,12 @@ function GroupListItem({
     onExpand(!isExpanded);
   };
 
-  const copyLink = () => {
+  /**
+   * @param {string} url
+   */
+  const copyLink = url => {
     try {
-      copyText(activityUrl);
+      copyText(url);
       toastMessenger.success(`Copied link for "${group.name}"`);
     } catch (err) {
       toastMessenger.error('Unable to copy link');
@@ -121,7 +124,7 @@ function GroupListItem({
             {activityUrl && (
               <li>
                 <MenuItem
-                  onClick={copyLink}
+                  onClick={() => copyLink(activityUrl)}
                   icon="copy"
                   isSubmenuItem={true}
                   label={copyLinkLabel}

src/sidebar/components/menu-item.js

@@ -91,7 +91,7 @@ export default function MenuItem({
   let focusTimer = null;
 
   let renderedIcon = null;
-  if (icon !== 'blank') {
+  if (icon && icon !== 'blank') {
     renderedIcon = iconIsUrl ? (
       <img className={iconClass} alt={iconAlt} src={icon} />
     ) : (