Fix a panic when diagnostics contain multiple links (#16601)

Follow up from #14518

Release Notes:

- Fixed a panic when diagnostics contain multiple links

crates/editor/src/editor_tests.rs

@@ -9090,6 +9090,43 @@ async fn go_to_prev_overlapping_diagnostic(
     "});
 }
 
+#[gpui::test]
+async fn test_diagnostics_with_links(cx: &mut TestAppContext) {
+    init_test(cx, |_| {});
+
+    let mut cx = EditorTestContext::new(cx).await;
+
+    cx.set_state(indoc! {"
+        fn func(abˇc def: i32) -> u32 {
+        }
+    "});
+    let project = cx.update_editor(|editor, _| editor.project.clone().unwrap());
+
+    cx.update(|cx| {
+        project.update(cx, |project, cx| {
+            project.update_diagnostics(
+                LanguageServerId(0),
+                lsp::PublishDiagnosticsParams {
+                    uri: lsp::Url::from_file_path("/root/file").unwrap(),
+                    version: None,
+                    diagnostics: vec![lsp::Diagnostic {
+                        range: lsp::Range::new(lsp::Position::new(0, 8), lsp::Position::new(0, 12)),
+                        severity: Some(lsp::DiagnosticSeverity::ERROR),
+                        message: "we've had problems with <https://link.one>, and <https://link.two> is broken".to_string(),
+                        ..Default::default()
+                    }],
+                },
+                &[],
+                cx,
+            )
+        })
+    }).unwrap();
+    cx.run_until_parked();
+    cx.update_editor(|editor, cx| hover_popover::hover(editor, &Default::default(), cx));
+    cx.run_until_parked();
+    cx.update_editor(|editor, _| assert!(editor.hover_state.diagnostic_popover.is_some()))
+}
+
 #[gpui::test]
 async fn go_to_hunk(executor: BackgroundExecutor, cx: &mut gpui::TestAppContext) {
     init_test(cx, |_| {});