Game crashes when sign is edited via commands while still being in the editor

When merging the data of a sign via data merge while someone is editing the sign manually the game of the person who's still editing it crashes.

After doing a little more research I found that the crash only occures if the command will effect any line besides the one being manually edited, which is the first. Also it only crashes once the "Done" button is clicked, as the thrown exception will show.

Also I got this message in the game output:

 
01:35:38
cft
main
fatal
 
Reported exception thrown! h: mouseClicked event handler at ckd.a(SourceFile:435) at cfv.a(SourceFile:83) at cfv$$Lambda$842/1525953166.invoke(Unknown Source) at org.lwjgl.glfw.GLFWMouseButtonCallbackI.callback(GLFWMouseButtonCallbackI.java:36) at org.lwjgl.system.JNI.invokeV(Native Method) at org.lwjgl.glfw.GLFW.glfwWaitEventsTimeout(GLFW.java:3108) at cgd.a(MainWindow.java:476) at cft.c(SourceFile:875) at cft.a(SourceFile:395) at net.minecraft.client.main.Main.main(SourceFile:144) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:497) at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) at net.minecraft.launchwrapper.Launch.main(Launch.java:28) Caused by: java.lang.NullPointerException at no.<init>(SourceFile:21) at clt.n(SourceFile:48) at cft.a(SourceFile:737) at clt.h(SourceFile:60) at clt.a(SourceFile:20) at clt$1.a(SourceFile:37) at cgu.mouseClicked(SourceFile:103) at chq.mouseClicked(SourceFile:46) at cfv.b(SourceFile:83) at cfv$$Lambda$1023/1035849634.run(Unknown Source) at ckd.a(SourceFile:430) ... 15 more