Exception with Pair class

[crashdemons]

09.08 15:17:43 [Server] WARN [CronPlugin] Task #244 for CronPlugin v0.0.1 generated an exception
09.08 15:17:43 [Server] INFO java.lang.NoClassDefFoundError: javafx/util/Pair
09.08 15:17:43 [Server] INFO at org.aztecmc.plugins.cron.CronPlugin$1.run(CronPlugin.java:40) ~[?:?]
09.08 15:17:43 [Server] INFO at org.bukkit.craftbukkit.v1_16_R1.scheduler.CraftTask.run(CraftTask.java:99) ~[patched_1.16.1.jar:git-Paper-133]
09.08 15:17:43 [Server] INFO at org.bukkit.craftbukkit.v1_16_R1.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:468) ~[patched_1.16.1.jar:git-Paper-133]
09.08 15:17:43 [Server] INFO at net.minecraft.server.v1_16_R1.MinecraftServer.b(MinecraftServer.java:1292) ~[patched_1.16.1.jar:git-Paper-133]
09.08 15:17:43 [Server] INFO at net.minecraft.server.v1_16_R1.DedicatedServer.b(DedicatedServer.java:377) ~[patched_1.16.1.jar:git-Paper-133]
09.08 15:17:43 [Server] INFO at net.minecraft.server.v1_16_R1.MinecraftServer.a(MinecraftServer.java:1213) ~[patched_1.16.1.jar:git-Paper-133]
09.08 15:17:43 [Server] INFO at net.minecraft.server.v1_16_R1.MinecraftServer.v(MinecraftServer.java:1001) ~[patched_1.16.1.jar:git-Paper-133]
09.08 15:17:43 [Server] INFO at net.minecraft.server.v1_16_R1.MinecraftServer.lambda$a$0(MinecraftServer.java:177) ~[patched_1.16.1.jar:git-Paper-133]
09.08 15:17:43 [Server] INFO at java.lang.Thread.run(Thread.java:834) [?:?]

1.16.1
09.08 15:19:09 [Server] INFO This server is running Paper version git-Paper-133 (MC: 1.16.1) (Implementing API version 1.16.1-R0.1-SNAPSHOT)

09.08 15:19:40 [Server] INFO CronPlugin version 0.0.1
09.08 15:19:40 [Server] INFO Author: AztecMC

I didn't dig any deeper into this, just saw it on Beta server suddenly