Program: 165580
Project: 165580
Buggy source:
@Override public void initGui(){
  super.initGui();
  list=new WidgetList(mc,this).setName("String_Node_Str").addSelectionEvent(new DefaultSelectionEvent(){
    @Override public void select(    Widget parent,    int index){
      setSelectedBlock(index);
    }
  }
);
  listDirty=0;
  Slider listSlider=new Slider(mc,this).setDesiredWidth(10).setVertical().setScrollableName("String_Node_Str");
  Panel listPanel=new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(3).setSpacing(1)).addChildren(list,listSlider);
  alarmModeChoiceLabel=new ChoiceLabel(mc,this).addChoices(RFMonitorMode.MODE_OFF.getDescription(),RFMonitorMode.MODE_LESS.getDescription(),RFMonitorMode.MODE_MORE.getDescription()).setDesiredWidth(60).setDesiredHeight(15).setTooltips("String_Node_Str","String_Node_Str").addChoiceEvent((parent,newChoice) -> changeAlarmMode(RFMonitorMode.getModeFromDescription(newChoice)));
  alarmModeChoiceLabel.setChoice(tileEntity.getAlarmMode().getDescription());
  alarmLabel=new ScrollableLabel(mc,this).setName("String_Node_Str").setSuffix("String_Node_Str").setDesiredWidth(30).setRealMinimum(0).setRealMaximum(100).setRealValue(tileEntity.getAlarmLevel()).addValueEvent((parent,newValue) -> changeAlarmValue(newValue));
  Slider alarmSlider=new Slider(mc,this).setDesiredHeight(15).setMinimumKnobSize(15).setHorizontal().setTooltips("String_Node_Str").setScrollableName("String_Node_Str");
  Panel alarmPanel=new Panel(mc,this).setLayout(new HorizontalLayout()).addChildren(alarmModeChoiceLabel,alarmSlider,alarmLabel).setDesiredHeight(20);
  Panel toplevel=new Panel(mc,this).setFilledRectThickness(2).setLayout(new VerticalLayout()).addChildren(listPanel,alarmPanel);
  toplevel.setBounds(new Rectangle(guiLeft,guiTop,xSize,ySize));
  window=new Window(this,toplevel);
  fromServer_clientAdjacentBlocks=new ArrayList<>();
  RFToolsMessages.INSTANCE.sendToServer(new PacketGetAdjacentTankBlocks(tileEntity.getPos()));
}

Fixed version (ground truth):
@Override public void initGui(){
  super.initGui();
  list=new WidgetList(mc,this).setName("String_Node_Str").addSelectionEvent(new DefaultSelectionEvent(){
    @Override public void select(    Widget<?> parent,    int index){
      setSelectedBlock(index);
    }
  }
);
  listDirty=0;
  Slider listSlider=new Slider(mc,this).setDesiredWidth(10).setVertical().setScrollableName("String_Node_Str");
  Panel listPanel=new Panel(mc,this).setLayout(new HorizontalLayout().setHorizontalMargin(3).setSpacing(1)).addChildren(list,listSlider);
  alarmModeChoiceLabel=new ChoiceLabel(mc,this).addChoices(RFMonitorMode.MODE_OFF.getDescription(),RFMonitorMode.MODE_LESS.getDescription(),RFMonitorMode.MODE_MORE.getDescription()).setDesiredWidth(60).setDesiredHeight(15).setTooltips("String_Node_Str","String_Node_Str").addChoiceEvent((parent,newChoice) -> changeAlarmMode(RFMonitorMode.getModeFromDescription(newChoice)));
  alarmModeChoiceLabel.setChoice(tileEntity.getAlarmMode().getDescription());
  alarmLabel=new ScrollableLabel(mc,this).setName("String_Node_Str").setSuffix("String_Node_Str").setDesiredWidth(30).setRealMinimum(0).setRealMaximum(100).setRealValue(tileEntity.getAlarmLevel()).addValueEvent((parent,newValue) -> changeAlarmValue(newValue));
  Slider alarmSlider=new Slider(mc,this).setDesiredHeight(15).setMinimumKnobSize(15).setHorizontal().setTooltips("String_Node_Str").setScrollableName("String_Node_Str");
  Panel alarmPanel=new Panel(mc,this).setLayout(new HorizontalLayout()).addChildren(alarmModeChoiceLabel,alarmSlider,alarmLabel).setDesiredHeight(20);
  Panel toplevel=new Panel(mc,this).setFilledRectThickness(2).setLayout(new VerticalLayout()).addChildren(listPanel,alarmPanel);
  toplevel.setBounds(new Rectangle(guiLeft,guiTop,xSize,ySize));
  window=new Window(this,toplevel);
  fromServer_clientAdjacentBlocks=new ArrayList<>();
  RFToolsMessages.INSTANCE.sendToServer(new PacketGetAdjacentTankBlocks(tileEntity.getPos()));
}