From 8d386cf5232f35a22afe20e6af7ca3117ba8db83 Mon Sep 17 00:00:00 2001
From: Brent Vatne <brentvatne@gmail.com>
Date: Mon, 19 Feb 2018 14:46:25 -0800
Subject: [PATCH] Fix regression in modular back button

---
 packages/react-navigation/src/views/Header/Header.js         | 5 +++++
 .../src/views/Header/ModularHeaderBackButton.js              | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/packages/react-navigation/src/views/Header/Header.js b/packages/react-navigation/src/views/Header/Header.js
index 5de1f1d3..77f63db2 100644
--- a/packages/react-navigation/src/views/Header/Header.js
+++ b/packages/react-navigation/src/views/Header/Header.js
@@ -277,6 +277,11 @@ class Header extends React.PureComponent {
     const { scene } = props;
     const { index, isStale, key } = scene;
 
+    // Never render a modular back button on the first screen in a stack.
+    if (index === 0) {
+      return;
+    }
+
     const offset = this.props.navigation.state.index - index;
 
     if (Math.abs(offset) > 2) {
diff --git a/packages/react-navigation/src/views/Header/ModularHeaderBackButton.js b/packages/react-navigation/src/views/Header/ModularHeaderBackButton.js
index 21b135f5..ee9b3908 100644
--- a/packages/react-navigation/src/views/Header/ModularHeaderBackButton.js
+++ b/packages/react-navigation/src/views/Header/ModularHeaderBackButton.js
@@ -42,7 +42,7 @@ class ModularHeaderBackButton extends React.PureComponent {
 
     // TODO: When we've sorted out measuring in the header, let's revisit this.
     // This is clearly a bad workaround.
-    if (backButtonTitle.length > 8) {
+    if (backButtonTitle && backButtonTitle.length > 8) {
       backButtonTitle = truncatedTitle;
     }