[Lens] Expression type on document can be null

Author: Maja Grubic

x-pack/legacy/plugins/lens/public/app_plugin/app.tsx

@@ -152,6 +152,7 @@ export function App({
 
   const isSaveable =
     lastKnownDoc &&
+    lastKnownDoc.expression &&
     lastKnownDoc.expression.length > 0 &&
     core.application.capabilities.visualize.save;
 

x-pack/legacy/plugins/lens/public/editor_frame_plugin/embeddable/expression_wrapper.tsx

@@ -13,7 +13,7 @@ import { ExpressionRenderer } from 'src/plugins/expressions/public';
 
 export interface ExpressionWrapperProps {
   ExpressionRenderer: ExpressionRenderer;
-  expression: string;
+  expression: string | null;
   context: {
     timeRange?: TimeRange;
     query?: Query;
@@ -29,7 +29,7 @@ export function ExpressionWrapper({
 }: ExpressionWrapperProps) {
   return (
     <I18nProvider>
-      {expression === '' ? (
+      {expression === null || expression === '' ? (
         <EuiFlexGroup direction="column" alignItems="center" justifyContent="center">
           <EuiFlexItem>
             <EuiIcon type="alert" color="danger" />

x-pack/legacy/plugins/lens/public/persistence/saved_object_store.ts

@@ -13,7 +13,7 @@ export interface Document {
   type?: string;
   visualizationType: string | null;
   title: string;
-  expression: string;
+  expression: string | null;
   state: {
     datasourceMetaData: {
       filterableIndexPatterns: Array<{ id: string; title: string }>;