Minecraft Launcher Crashing?! 1.12.1

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[18:52:37 INFO]: Job 'Version & Libraries' finished successfully (took 0:00:01.989)
[18:52:37 INFO]: Launching game
[18:52:37 INFO]: Unpacking natives to /Users/jonnyheather/Library/Application Support/minecraft/versions/1.12.1/1.12.1-natives-1503510757563157000
[18:52:37 INFO]: Launching in /Users/jonnyheather/Library/Application Support/minecraft
[18:52:37 INFO]: Half command: /Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/bin/java -Xdock:icon=/Users/jonnyheather/Library/Application Support/minecraft/assets/objects/99/991b421dfd401f115241601b2b373140a8d78572 -Xdock:name=Minecraft -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M -Djava.library.path=/Users/jonnyheather/Library/Application Support/minecraft/versions/1.12.1/1.12.1-natives-1503510757563157000 -cp /Users/jonnyheather/Library/Application Support/minecraft/libraries/com/mojang/patchy/1.1/patchy-1.1.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/oshi-project/oshi-core/1.1/oshi-core-1.1.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/net/java/dev/jna/jna/4.4.0/jna-4.4.0.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/net/java/dev/jna/platform/3.4.0/platform-3.4.0.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/ibm/icu/icu4j-core-mojang/51.2/icu4j-core-mojang-51.2.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/net/sf/jopt-simple/jopt-simple/5.0.3/jopt-simple-5.0.3.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/paulscode/codecjorbis/20101023/codecjorbis-20101023.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/paulscode/codecwav/20101023/codecwav-20101023.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/paulscode/libraryjavasound/20101123/libraryjavasound-20101123.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/paulscode/librarylwjglopenal/20100824/librarylwjglopenal-20100824.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/paulscode/soundsystem/20120107/soundsystem-20120107.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/io/netty/netty-all/4.1.9.Final/netty-all-4.1.9.Final.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/google/guava/guava/21.0/guava-21.0.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/apache/commons/commons-lang3/3.5/commons-lang3-3.5.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/commons-io/commons-io/2.5/commons-io-2.5.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/commons-codec/commons-codec/1.10/commons-codec-1.10.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/net/java/jinput/jinput/2.0.5/jinput-2.0.5.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/net/java/jutils/jutils/1.0.0/jutils-1.0.0.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/google/code/gson/gson/2.8.0/gson-2.8.0.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/mojang/authlib/1.5.25/authlib-1.5.25.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/mojang/realms/1.10.17/realms-1.10.17.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/apache/commons/commons-compress/1.8.1/commons-compress-1.8.1.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/apache/httpcomponents/httpclient/4.3.3/httpclient-4.3.3.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/commons-logging/commons-logging/1.1.3/commons-logging-1.1.3.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/apache/httpcomponents/httpcore/4.3.2/httpcore-4.3.2.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/it/unimi/dsi/fastutil/7.1.0/fastutil-7.1.0.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/apache/logging/log4j/log4j-api/2.8.1/log4j-api-2.8.1.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/apache/logging/log4j/log4j-core/2.8.1/log4j-core-2.8.1.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/lwjgl/lwjgl/lwjgl/2.9.2-nightly-20140822/lwjgl-2.9.2-nightly-20140822.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/org/lwjgl/lwjgl/lwjgl_util/2.9.2-nightly-20140822/lwjgl_util-2.9.2-nightly-20140822.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/com/mojang/text2speech/1.10.3/text2speech-1.10.3.jar:/Users/jonnyheather/Library/Application Support/minecraft/libraries/ca/weblite/java-objc-bridge/1.0.0/java-objc-bridge-1.0.0.jar:/Users/jonnyheather/Library/Application Support/minecraft/versions/1.12.1/1.12.1.jar net.minecraft.client.main.Main
[18:52:37 INFO]: Looking for orphaned versions to clean up...
[18:52:37 FATAL]: Fatal error launching game. Report this to http://bugs.mojang.com please!
java.io.FileNotFoundException: File '/Users/jonnyheather/Library/Application Support/minecraft/assets/indexes/legacy.json' does not exist
	at org.apache.commons.io.FileUtils.openInputStream(FileUtils.java:299) ~[launcher.jar:1.6.61]
	at org.apache.commons.io.FileUtils.readFileToString(FileUtils.java:1711) ~[launcher.jar:1.6.61]
	at net.minecraft.launcher.Launcher.cleanupOrphanedAssets(Launcher.java:254) ~[launcher.jar:1.6.61]
	at net.minecraft.launcher.Launcher.performCleanups(Launcher.java:496) ~[launcher.jar:1.6.61]
	at net.minecraft.launcher.game.MinecraftGameRunner.launchGame(MinecraftGameRunner.java:231) ~[launcher.jar:1.6.61]
	at com.mojang.launcher.game.runner.AbstractGameRunner.onDownloadJobFinished(AbstractGameRunner.java:186) [launcher.jar:1.6.61]
	at com.mojang.launcher.updater.download.DownloadJob.popAndDownload(DownloadJob.java:122) [launcher.jar:1.6.61]
	at com.mojang.launcher.updater.download.DownloadJob.access$000(DownloadJob.java:12) [launcher.jar:1.6.61]
	at com.mojang.launcher.updater.download.DownloadJob$1.run(DownloadJob.java:86) [launcher.jar:1.6.61]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [?:1.7.0_71]
	at java.util.concurrent.FutureTask.run(FutureTask.java:262) [?:1.7.0_71]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [?:1.7.0_71]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [?:1.7.0_71]
	at java.lang.Thread.run(Thread.java:745) [?:1.7.0_71]
[18:52:38 ERROR]: Game ended with bad state (exit code 1)
[18:52:38 INFO]: Ignoring visibility rule and showing launcher due to a game crash
[18:52:38 INFO]: Deleting /Users/jonnyheather/Library/Application Support/minecraft/versions/1.12.1/1.12.1-natives-1503510757563157000

From MCL-1905:

Potential Cause

This Issue is potentially caused by Antivirus Programs and/or Firewall Programs, especially Norton. Try disabling it while launching Minecraft and see if it helps. If it does maybe consider using a different Antivirus Program.

Also, this issue might easily be fixed by reinstalling Minecraft.

You could also try updating Java.
If none of this works, try community support.

Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki

Thank you! Did everything you suggested and seems to be working just fine now!