Bug 1533945 - Handle selectedFrame being unknown. r=jlast

Differential Revision: https://phabricator.services.mozilla.com/D22812

--HG--
extra : moz-landing-system : lando

devtools/client/debugger/new/src/components/SecondaryPanes/Scopes.js

@@ -113,7 +113,11 @@ class Scopes extends PureComponent<Props, State> {
   renderMapScopes() {
     const { selectedFrame, shouldMapScopes } = this.props;
 
-    if (!features.mapScopes || isGeneratedId(selectedFrame.location.sourceId)) {
+    if (
+      !features.mapScopes ||
+      !selectedFrame ||
+      isGeneratedId(selectedFrame.location.sourceId)
+    ) {
       return null;
     }