Handle invalid issues

options/locale/locale_en-US.ini

@@ -108,6 +108,7 @@ occurred = An error has occurred
 report_message = If you are sure this is a Gitea bug, please search for issue on <a href="https://github.com/go-gitea/gitea/issues">GitHub</a> and open new issue if necessary.
 missing_csrf = Bad Request: no CSRF token present
 invalid_csrf = Bad Request: Invalid CSRF token
+issue_not_found = The referenced issue couldn't be found.
 
 [startpage]
 app_desc = A painless, self-hosted Git service

templates/base/head.tmpl

@@ -49,6 +49,8 @@
 			i18n: {
 				copy_success: '{{.i18n.Tr "copy_success"}}',
 				copy_error: '{{.i18n.Tr "copy_error"}}',
+				error_occurred: '{{.i18n.Tr "error.occurred"}}',
+				issue_not_found: '{{.i18n.Tr "error.issue_not_found"}}',
 			}
 		};
 		{{/* in case some pages don't render the pageData, we make sure it is an object to prevent null access */}}

web_src/js/components/ContextPopup.vue

@@ -16,13 +16,18 @@
         </div>
       </div>
     </div>
+    <div v-if="!loading && issue === null">
+      <p><small>{{ errorTitle }}</small></p>
+      <p>{{ errorBody }}</p>
+    </div>
   </div>
 </template>
 
 <script>
 import {SvgIcon} from '../svg.js';
 
 const {appSubUrl} = window.config;
+const {issue_not_found, error_occurred} = window.config.i18n;
 
 // NOTE: see models/issue_label.go for similar implementation
 const srgbToLinear = (color) => {
@@ -53,6 +58,14 @@ export default {
   }),
 
   computed: {
+    errorTitle() {
+      return error_occurred;
+    },
+
+    errorBody() {
+      return issue_not_found;
+    },
+
     createdAt() {
       return new Date(this.issue.created_at).toLocaleDateString(undefined, {year: 'numeric', month: 'short', day: 'numeric'});
     },
@@ -120,6 +133,13 @@ export default {
             callback();
           }
         });
+      }).fail(() => {
+        this.loading = false;
+        this.$nextTick(() => {
+          if (callback) {
+            callback();
+          }
+        });
       });
     }
   }