Fix getUsefulTabs to handle cases where the first sample in a thread …

src/selectors/per-thread/composed.js

@@ -26,8 +26,6 @@ import type {
   StackTimingByDepth,
 } from '../../profile-logic/stack-timing';
 
-import { ensureExists } from '../../utils/flow';
-
 /**
  * Infer the return type from the getStackAndSampleSelectorsPerThread function. This
  * is done that so that the local type definition with `Selector<T>` is the canonical
@@ -92,8 +90,15 @@ export function getComposedSelectorsPerThread(
       }
       let hasSamples = samples.length > 0 && stackTable.length > 0;
       if (hasSamples) {
-        const stackIndex = ensureExists(samples.stack[0]);
-        if (stackTable.prefix[stackIndex] === null) {
+        // Find at least one non-null stack.
+        const stackIndex = samples.stack.find((stack) => stack !== null);
+        if (
+          stackIndex === undefined ||
+          stackIndex === null // We know that it can't be null at this point, but Flow doesn't.
+        ) {
+          // All samples were null.
+          hasSamples = false;
+        } else if (stackTable.prefix[stackIndex] === null) {
           // There's only a single stack frame, check if it's '(root)'.
           const frameIndex = stackTable.frame[stackIndex];
           const funcIndex = frameTable.func[frameIndex];

src/test/store/useful-tabs.test.js

@@ -116,4 +116,18 @@ describe('getUsefulTabs', function () {
       'marker-table',
     ]);
   });
+
+  it('works when the first sample is null', () => {
+    const { profile } = getProfileFromTextSamples('A  B');
+    profile.threads[0].samples.stack[0] = null;
+
+    const { getState } = storeWithProfile(profile);
+    expect(selectedThreadSelectors.getUsefulTabs(getState())).toEqual([
+      'calltree',
+      'flame-graph',
+      'stack-chart',
+      'marker-chart',
+      'marker-table',
+    ]);
+  });
 });