Fixes NPE in page.columns

Summary: From stack trace seems that we don't always get columns from the plugin, in which case we can't display anything

Reviewed By: lawrencelomax

Differential Revision: D33981285

fbshipit-source-id: 0d6bf2d9364f7dac93e2179e372308da51699bd3

desktop/plugins/public/databases/DatabasesPlugin.tsx

@@ -216,7 +216,7 @@ const DataTable = React.memo(
     currentStructure: Structure | null;
     onRowEdited: (changes: {[key: string]: string | null}) => void;
   }) =>
-    page ? (
+    page && page.columns ? (
       <Layout.Horizontal grow>
         <ManagedTable
           tableKey={`databases-${page.databaseId}-${page.table}`}