Add NPE check when setting back button

android/app/src/main/java/com/reactnativenavigation/utils/ContextProvider.java

 package com.reactnativenavigation.utils;
 
+import android.support.annotation.Nullable;
+
 import com.reactnativenavigation.activities.BaseReactActivity;
 
 import java.lang.ref.WeakReference;
     /**
      * Returns the currently resumed activity or {@code null} if there is none.
      */
-    public static BaseReactActivity getActivityContext() {
+    public static @Nullable BaseReactActivity getActivityContext() {
         return sActivityWR != null ? sActivityWR.get() : null;
     }
 

android/app/src/main/java/com/reactnativenavigation/views/RnnToolBar.java

 
     @SuppressWarnings({"ConstantConditions"})
     public void setNavUpButton(Screen screen) {
-        ActionBar actionBar = ContextProvider.getActivityContext().getSupportActionBar();
+        BaseReactActivity context = ContextProvider.getActivityContext();
+        if (context == null) {
+            return;
+        }
+
+        ActionBar actionBar = context.getSupportActionBar();
         if (actionBar == null) {
             return;
         }
             navArrow = (DrawerArrowDrawable) this.getNavigationIcon();
         } else {
             if (isBack && !screen.backButtonHidden) {
-                navArrow = new DrawerArrowDrawable(ContextProvider.getActivityContext());
+                navArrow = new DrawerArrowDrawable(context);
             } else if (hasDrawer) {
                 navIcon = mDrawerIcon;
             }