Program: 165587
Project: 165587
Buggy source:
@Override public void initGui(){
  super.initGui();
  players=new WidgetList(mc,this).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  Slider allowedPlayerSlider=new Slider(mc,this).setDesiredWidth(10).setVertical().setScrollableName("String_Node_Str");
  Panel allowedPlayersPanel=new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(3).setSpacing(1)).addChildren(players,allowedPlayerSlider).setLayoutHint(new PositionalLayout.PositionalHint(72,5,SECURITYMANAGER_WIDTH - 76,96));
  nameField=new TextField(mc,this).setDesiredHeight(15).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  Widget addButton=new Button(mc,this).setText("String_Node_Str").setDesiredHeight(14).setDesiredWidth(34).setTooltips("String_Node_Str").setEnabledFlags("String_Node_Str").setName("String_Node_Str").setChannel("String_Node_Str");
  Widget delButton=new Button(mc,this).setText("String_Node_Str").setDesiredHeight(14).setDesiredWidth(34).setTooltips("String_Node_Str","String_Node_Str").setEnabledFlags("String_Node_Str").setName("String_Node_Str").setChannel("String_Node_Str");
  Panel buttonPanel=new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(3).setSpacing(1)).addChildren(nameField,addButton,delButton).setDesiredHeight(16).setLayoutHint(new PositionalLayout.PositionalHint(72,100,SECURITYMANAGER_WIDTH - 76,14));
  channelNameField=new TextField(mc,this).setLayoutHint(8,27,60,14).addTextEvent((parent,newText) -> updateChannelName()).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  blacklistMode=new ImageChoiceLabel(mc,this).setLayoutHint(10,44,16,16).setTooltips("String_Node_Str").addChoiceEvent((parent,newChoice) -> updateSettings()).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  blacklistMode.addChoice("String_Node_Str","String_Node_Str",guiElements,15 * 16,32);
  blacklistMode.addChoice("String_Node_Str","String_Node_Str",guiElements,14 * 16,32);
  Panel toplevel=new Panel(mc,this).setBackground(iconLocation).setLayout(new PositionalLayout()).addChildren(allowedPlayersPanel,buttonPanel,channelNameField,blacklistMode);
  toplevel.setBounds(new Rectangle(guiLeft,guiTop,xSize,ySize));
  window=new Window(this,toplevel);
  Keyboard.enableRepeatEvents(true);
  window.event("String_Node_Str",(source,params) -> addPlayer());
  window.event("String_Node_Str",(source,params) -> delPlayer());
  channelFromServer=null;
}

Fixed version (ground truth):
@Override public void initGui(){
  super.initGui();
  players=new WidgetList(mc,this).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  Slider allowedPlayerSlider=new Slider(mc,this).setDesiredWidth(10).setVertical().setScrollableName("String_Node_Str");
  Panel allowedPlayersPanel=new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(3).setSpacing(1)).addChildren(players,allowedPlayerSlider).setLayoutHint(new PositionalLayout.PositionalHint(72,5,SECURITYMANAGER_WIDTH - 76,96));
  nameField=new TextField(mc,this).setDesiredHeight(15).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  Widget<?> addButton=new Button(mc,this).setText("String_Node_Str").setDesiredHeight(14).setDesiredWidth(34).setTooltips("String_Node_Str").setEnabledFlags("String_Node_Str").setName("String_Node_Str").setChannel("String_Node_Str");
  Widget<?> delButton=new Button(mc,this).setText("String_Node_Str").setDesiredHeight(14).setDesiredWidth(34).setTooltips("String_Node_Str","String_Node_Str").setEnabledFlags("String_Node_Str").setName("String_Node_Str").setChannel("String_Node_Str");
  Panel buttonPanel=new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(3).setSpacing(1)).addChildren(nameField,addButton,delButton).setDesiredHeight(16).setLayoutHint(new PositionalLayout.PositionalHint(72,100,SECURITYMANAGER_WIDTH - 76,14));
  channelNameField=new TextField(mc,this).setLayoutHint(8,27,60,14).addTextEvent((parent,newText) -> updateChannelName()).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  blacklistMode=new ImageChoiceLabel(mc,this).setLayoutHint(10,44,16,16).setTooltips("String_Node_Str").addChoiceEvent((parent,newChoice) -> updateSettings()).setName("String_Node_Str").setEnabledFlags("String_Node_Str");
  blacklistMode.addChoice("String_Node_Str","String_Node_Str",guiElements,15 * 16,32);
  blacklistMode.addChoice("String_Node_Str","String_Node_Str",guiElements,14 * 16,32);
  Panel toplevel=new Panel(mc,this).setBackground(iconLocation).setLayout(new PositionalLayout()).addChildren(allowedPlayersPanel,buttonPanel,channelNameField,blacklistMode);
  toplevel.setBounds(new Rectangle(guiLeft,guiTop,xSize,ySize));
  window=new Window(this,toplevel);
  Keyboard.enableRepeatEvents(true);
  window.event("String_Node_Str",(source,params) -> addPlayer());
  window.event("String_Node_Str",(source,params) -> delPlayer());
  channelFromServer=null;
}