My Gintro program is crashing

[adnan]

Hello, I was trying to create a small Gtk program for my personal use. It has a 
glib timeout function that reads data from a few spinbuttons, comboboxtext and 
an entry periodically and automatically refreshes a couple of labels. Here is 
the app itself:
    
    
    import gintro/[gtk, glib, gobject, gio]
    import strutils
    
    proc updateIntervalResult(builder: Builder): bool =
        result = true
        let
          startAM = builder.getComboBoxText("start_am_pm").getActiveText() == 
"AM"
          startHour = builder.getSpinButton("start_hour").getValue() + (if 
startAM: 0 else: 12)
          startMin = builder.getSpinButton("start_minute").getValue()
          
          endAM = builder.getComboBoxText("end_am_pm").getActiveText() == "AM"
          endHour = builder.getSpinButton("end_hour").getValue() + (if endAM: 0 
else: 12)
          endMin = builder.getSpinButton("end_minute").getValue()
        
        let interval = (endHour * 60 + endMin - startHour * 60 - startMin) / 60
        if interval < 0.0:
          builder.getLabel("interval_label").setLabel("-")
          builder.getLabel("currency_label").setLabel("-")
        else:
          let intervalFormat = interval.formatFloat(ffDecimal, 2)
          builder.getLabel("interval_label").setLabel(intervalFormat)
          if builder.getEntry("rate").getTextLength() > uint16(0):
            try:
              let rateStr = builder.getEntry("rate").getText()
              let rate = rateStr.parseFloat()
              builder.getLabel("currency_label").setLabel(cstring("$" & 
(interval * rate).formatFloat(ffDecimal, 2)))
            except:
              echo "parsing currency failed"
              builder.getLabel("currency_label").setLabel("$0.00")
          else:
              builder.getLabel("currency_label").setLabel(" ")
    
    
    proc appActivate(app: Application) =
      let builder = newBuilderFromFile("resources/ui.glade")
      let appWindow = builder.getApplicationWindow("aw")
      appWindow.setApplication(app)
      timeoutAdd(400, updateIntervalResult, builder)
      appWindow.showAll()
    
    when isMainModule:
      let app = newApplication("com.gitlab.adnan338.Interval")
      app.connect("activate", appActivate)
      discard run(app)
    
    
    Run

Here is the glade file: <https://pastebin.com/hAFqWm8b>

However the app crashes after a few seconds by itself after entering some data 
saying:

Error: unhandled exception: 
/home/adnan338/.nimble/pkgs/gintro-0.8.3/gintro/gtk.nim(61531, 11) `result.impl 
== gobj` [AssertionError]

What could be causing this? Thanks in advance.