Launcher crashes after system updated to Windows 8.1

[19:51:34 FATAL]: Fatal error launching game. Report this to http://bugs.mojang.com please!
java.io.FileNotFoundException: File 'C:\Users\rob\AppData\Roaming\.minecraft\assets\indexes\legacy.json' does not exist
at org.apache.commons.io.FileUtils.openInputStream(FileUtils.java:299) ~[launcher.jar:1.6.5]
at org.apache.commons.io.FileUtils.readFileToString(FileUtils.java:1711) ~[launcher.jar:1.6.5]
at net.minecraft.launcher.Launcher.cleanupOrphanedAssets(Launcher.java:253) ~[launcher.jar:1.6.5]
at net.minecraft.launcher.Launcher.performCleanups(Launcher.java:495) ~[launcher.jar:1.6.5]
at net.minecraft.launcher.game.MinecraftGameRunner.launchGame(MinecraftGameRunner.java:221) ~[launcher.jar:1.6.5]
at com.mojang.launcher.game.runner.AbstractGameRunner.onDownloadJobFinished(AbstractGameRunner.java:186) [launcher.jar:1.6.5]
at com.mojang.launcher.updater.download.DownloadJob.popAndDownload(DownloadJob.java:122) [launcher.jar:1.6.5]
at com.mojang.launcher.updater.download.DownloadJob.access$000(DownloadJob.java:12) [launcher.jar:1.6.5]
at com.mojang.launcher.updater.download.DownloadJob$1.run(DownloadJob.java:86) [launcher.jar:1.6.5]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_25]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_25]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [?:1.8.0_25]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [?:1.8.0_25]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_25]
[19:52:32 ERROR]: Game ended with bad state (exit code 1)
[19:52:32 INFO]: Ignoring visibility rule and showing launcher due to a game crash
[19:52:32 INFO]: Deleting C:\Users\rob\AppData\Roaming\.minecraft\versions\1.8.1\1.8.1-natives-3063837067260
[19:52:32 WARN]: Couldn't delete C:\Users\rob\AppData\Roaming\.minecraft\versions\1.8.1\1.8.1-natives-3063837067260 - scheduling for deletion upon exit