Add warning when scrollRef does not have a scrollTo method

Summary:
Add a `console.warn()` call when calling `_scrollRef.scrollTo`, because `scrollTo` is not guaranteed to exist on `_scrollRef`.

Context:
`VirtualizedList` holds `_scrollRef`, which is usually a reference to a `ScrollView` component. However, there are several cases where it holds a `View` or other type of component instead.

A custom component can be passed in to `renderScrollComponent`, and then `_scrollRef` will point to that custom component. Additionally, if two VirtualizedLists are nested with the same orientation, `_defaultRenderScrollComponent` will return a View instead of a ScrollView.

Due to these possibilities, `_scrollRef` is not guaranteed to have a `scrollTo` method.

Changelog: [General] [Added] - Add warning when scrollRef does not have a scrollTo method

Reviewed By: JoshuaGross, TheSavior

Differential Revision: D21386842

fbshipit-source-id: 01e167e0ae0edea8f29853e8b242ce88a5103b49

Author: kacieb

Libraries/Lists/VirtualizedList.js

@@ -374,6 +374,15 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       return;
     }
 
+    if (this._scrollRef.scrollTo == null) {
+      console.warn(
+        'No scrollTo method provided. This may be because you have two nested ' +
+          'VirtualizedLists with the same orientation, or because you are ' +
+          'using a custom component that does not implement scrollTo.',
+      );
+      return;
+    }
+
     this._scrollRef.scrollTo(
       this.props.horizontal ? {x: offset, animated} : {y: offset, animated},
     );
@@ -437,6 +446,15 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       return;
     }
 
+    if (this._scrollRef.scrollTo == null) {
+      console.warn(
+        'No scrollTo method provided. This may be because you have two nested ' +
+          'VirtualizedLists with the same orientation, or because you are ' +
+          'using a custom component that does not implement scrollTo.',
+      );
+      return;
+    }
+
     this._scrollRef.scrollTo(
       horizontal ? {x: offset, animated} : {y: offset, animated},
     );
@@ -478,6 +496,15 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       return;
     }
 
+    if (this._scrollRef.scrollTo == null) {
+      console.warn(
+        'No scrollTo method provided. This may be because you have two nested ' +
+          'VirtualizedLists with the same orientation, or because you are ' +
+          'using a custom component that does not implement scrollTo.',
+      );
+      return;
+    }
+
     this._scrollRef.scrollTo(
       this.props.horizontal ? {x: offset, animated} : {y: offset, animated},
     );