Show first component stack in context warning

Author: gaearon

packages/react-reconciler/src/ReactStrictModeWarnings.js

@@ -324,16 +324,19 @@ if (__DEV__) {
   ReactStrictModeWarnings.flushLegacyContextWarning = () => {
     ((pendingLegacyContextWarning: any): FiberToFiberComponentsMap).forEach(
       (fiberArray: FiberArray, strictRoot) => {
+        if (fiberArray.length === 0) {
+          return;
+        }
+        const firstFiber = fiberArray[0];
+
         const uniqueNames = new Set();
         fiberArray.forEach(fiber => {
           uniqueNames.add(getComponentName(fiber.type) || 'Component');
           didWarnAboutLegacyContext.add(fiber.type);
         });
 
         const sortedNames = setToSortedString(uniqueNames);
-        const strictRootComponentStack = getStackByFiberInDevAndProd(
-          strictRoot,
-        );
+        const firstComponentStack = getStackByFiberInDevAndProd(firstFiber);
 
         console.error(
           'Legacy context API has been detected within a strict-mode tree.' +
@@ -343,7 +346,7 @@ if (__DEV__) {
             '\n\nLearn more about this warning here: https://fb.me/react-legacy-context' +
             '%s',
           sortedNames,
-          strictRootComponentStack,
+          firstComponentStack,
         );
       },
     );