Error loading images using CachedNetworkImage #39

I/flutter (27693): The following NoSuchMethodError was thrown resolving an image codec:
I/flutter (27693): The method 'containsKey' was called on null.
I/flutter (27693): Receiver: null
I/flutter (27693): Tried calling:
I/flutter (27693): containsKey("https://www.theimageurl.com/theimage.jpg")
I/flutter (27693):
I/flutter (27693): When the exception was thrown, this was the stack:
I/flutter (27693): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:46:5)
I/flutter (27693): #1 CacheManager.getFile (package:flutter_cache_manager/flutter_cache_manager.dart:190:21)
I/flutter (27693):
I/flutter (27693): #2 CachedNetworkImageProvider._loadAsync (package:cached_network_image/cached_network_image.dart:472:35)
I/flutter (27693):
I/flutter (27693): #3 CachedNetworkImageProvider.load (package:cached_network_image/cached_network_image.dart:462:16)
I/flutter (27693): #4 ImageProvider.resolve.. (package:flutter/src/painting/image_provider.dart:266:86)
I/flutter (27693): #5 ImageCache.putIfAbsent (package:flutter/src/painting/image_cache.dart:143:20)
I/flutter (27693): #6 ImageProvider.resolve. (package:flutter/src/painting/image_provider.dart:266:63)
I/flutter (27693): #7 SynchronousFuture.then (package:flutter/src/foundation/synchronous_future.dart:38:29)
I/flutter (27693): #8 ImageProvider.resolve (package:flutter/src/painting/image_provider.dart:264:30)
I/flutter (27693): #9 _ImageProviderResolver.resolve (package:cached_network_image/cached_network_image.dart:193:29)
I/flutter (27693): #10 _CachedNetworkImageState._resolveImage (package:cached_network_image/cached_network_image.dart:284:20)
I/flutter (27693): #11 _CachedNetworkImageState.didChangeDependencies (package:cached_network_image/cached_network_image.dart:261:5)
I/flutter (27693): #12 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3780:12)
I/flutter (27693): #13 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3630:5)
...

Many images load fine, but on occasion, some fail. The error is thrown from CacheManager on the third line of this function.

Future<File> getFile(String url, {Map<String, String> headers}) async {
    String log = "[Flutter Cache Manager] Loading $url";

    if (!_cacheData.containsKey(url)) {
      await synchronized(_lock, () {
        if (!_cacheData.containsKey(url)) {
          _cacheData[url] = new CacheObject(url);
        }
      });
    }
    ...