fix a couple of blockly bugs in the code eval embed (#10588)

Author: riknoll

pxtblocks/layout.ts

@@ -452,7 +452,7 @@ async function expandImagesAsync(xsg: Document): Promise<void> {
     const linkedSvgImages = images
         .filter(image => {
             const href = image.getAttribute("href");
-            return href.endsWith(".svg") &&
+            return href?.endsWith(".svg") &&
                 (
                     href.startsWith("/") ||
                     href.startsWith(pxt.webConfig.cdnUrl)

webapp/src/blocks.tsx

@@ -220,7 +220,7 @@ export class Editor extends toolboxeditor.ToolboxEditor {
             this.loadingXml = true;
 
             const flyout = this.editor.getFlyout() as pxtblockly.CachingFlyout;
-            flyout.clearBlockCache();
+            flyout?.clearBlockCache();
 
             const loadingDimmer = document.createElement("div");
             loadingDimmer.className = "ui active dimmer";