Colorable#getColor() retuns a @Nullable DyeColor: https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/material/Colorable.html#getColor()
As per description:
This may be null to represent the default color of an object, if the object has a special default color (e.g Shulkers).

If you look at the ShulkerBox interface though, you'll notice that getColor() is marked as @NotNull DyeColor: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/browse/src/main/java/org/bukkit/block/ShulkerBox.java

If you look into Spigot-Server/src/main/java/org/bukkit/craftbukkit/block/CraftShulkerBox.java, you'll notice:

return DyeColor.getByWoolData((byte) ((BlockShulkerBox) block).color.getColorIndex());

This is the cause of NPE, since block.color will be null in case of shulkerboxes without color.

This can easily be replicate with grakkit by standing on a regular shulkerbox and running:

/js self.getWorld().getBlockAt(self.getLocation().add(0, -1, 0)).getState().getColor()

If you use a modern java like Java16, you'll see:

java.lang.NullPointerException: Cannot invoke "net.minecraft.server.v1_16_R3.EnumColor.getColorIndex()" because "block.color" is null