Correct an issue where "dashboard.panel.edit" fatals in the API console

Summary:
Ref T13650. Currently, viewing the API console help page for this method fatals because it constructs a generic, untyped panel.

As a step toward improving this, generate a concrete panel type instead. This isn't the best possible fix, see T13650 for discussion.

Test Plan: Viewed "dashboard.panel.edit" API page, now saw a usable page.

Maniphest Tasks: T13650

Differential Revision: https://secure.phabricator.com/D21668

src/applications/dashboard/editor/PhabricatorDashboardPanelEditEngine.php

@@ -71,6 +71,17 @@ final class PhabricatorDashboardPanelEditEngine
     return $panel;
   }
 
+  protected function newEditableObjectForDocumentation() {
+    $panel = parent::newEditableObjectForDocumentation();
+
+    $text_type = id(new PhabricatorDashboardTextPanelType())
+      ->getPanelTypeKey();
+
+    $panel->setPanelType($text_type);
+
+    return $panel;
+  }
+
   protected function newObjectQuery() {
     return new PhabricatorDashboardPanelQuery();
   }