EnhancedDebugger.addReadPacketToTable(): avoid NPE when messagesTable is null on after cancel()

[stokito]

I started Spark with a debugger and during the Spark exiting there was a few exceptions:

java.lang.NullPointerException: Cannot invoke "javax.swing.table.DefaultTableModel.getRowCount()" because "this.this$0.messagesTable" is null
	at org.jivesoftware.smackx.debugger.EnhancedDebugger$20.run(EnhancedDebugger.java:806)

So here is a null check to avoid such an error. Some description from AI:

To provide a long-term fix, we need to add a null check before accessing messagesTable inside the EnhancedDebugger class. Since this is happening inside a run() method (likely an anonymous inner class or lambda passed to SwingUtilities.invokeLater), we need to ensure the debugger's UI state is valid before proceeding.