Glide Version/Integration library (if any): 3.5.2
Device/Android Version: N/A
Issue details/Repro steps:

DiskCache.Factory.build() method JavaDoc says

Returns a new disk cache, or {@code null} if no disk cache could be created.

and Engine.LazyDiskCacheProvider.getDiskCache() just returns the value,
then DecodeJob uses that return value in a chain: diskCacheProvider.getDiskCache().get|put|delete().

Meaning a custom Disk Cache Factory may fail:

public static class GlideSetup implements GlideModule {
    @Override public void applyOptions(final Context context, GlideBuilder builder) {
        builder.setDiskCache(new DiskCache.Factory() {
            @Override public DiskCache build() {
                File cacheDir = Paths.getGlideCacheDirectory(context);
                if (cacheDir == null
                        || (!cacheDir.mkdirs() && (!cacheDir.exists() || !cacheDir.isDirectory()))) {
                    // Note that the condition is almost the same as Glide.getPhotoCacheDir
                    return null;
                }
                return DiskLruCacheWrapper.get(cacheDir, 250 * 1024 * 1024);
            }
        });
    }
    @Override public void registerComponents(Context context, Glide glide) {}
}

In case the DiskCache.Factory can't create a cache (no available directory, the a App will crash with an NPE in Glide, however it could probably work just fine without a disk cache, maybe a little slower.

Probably the

        if (diskCache == null) {
            diskCache = new DiskCacheAdapter();
        }

lines in InternalCacheDiskCacheFactory.build should be moved to LazyDiskCacheProvider, so the fallback applies to all caches, not just the Glide built-in one.

PS: Also note that's an awful lot of lines, just to move the disk cache to another directory.