Comment 6 for bug 914554

Confirming, I've reproduced the bug using the eyedropper with a PNG at 800% zoom.

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object
  at Pinta.Core.GdkExtensions.Invalidate (Gdk.Window w) [0x00000] in <filename unknown>:0
  at Pinta.Gui.Widgets.ColorPaletteWidget.Palette_ColorChanged (System.Object sender, System.EventArgs e) [0x00000] in <filename unknown>:0
  at Pinta.Core.PaletteManager.OnPrimaryColorChanged () [0x00000] in <filename unknown>:0
  at Pinta.Core.PaletteManager.set_PrimaryColor (Color value) [0x00000] in <filename unknown>:0
  at Pinta.Tools.ColorPickerTool.OnMouseDown (Gtk.DrawingArea canvas, Gtk.ButtonPressEventArgs args, PointD point) [0x00000] in <filename unknown>:0
  at Pinta.Core.BaseTool.DoMouseDown (Gtk.DrawingArea canvas, Gtk.ButtonPressEventArgs args, PointD point) [0x00000] in <filename unknown>:0
  at Pinta.Gui.Widgets.PintaCanvas.<PintaCanvas>m__E (System.Object sender, Gtk.ButtonPressEventArgs e) [0x00000] in <filename unknown>:0
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0
  --- End of inner exception stack trace ---
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0
  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <filename unknown>:0
  at System.Delegate.DynamicInvokeImpl (System.Object[] args) [0x00000] in <filename unknown>:0
  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) [0x00000] in <filename unknown>:0
  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in <filename unknown>:0
  at GLib.Signal.ClosureInvokedCB (System.Object o, GLib.ClosureInvokedArgs args) [0x00000] in <filename unknown>:0
  at GLib.SignalClosure.Invoke (GLib.ClosureInvokedArgs args) [0x00000] in <filename unknown>:0
  at GLib.SignalClosure.MarshalCallback (IntPtr raw_closure, IntPtr return_val, UInt32 n_param_vals, IntPtr param_values, IntPtr invocation_hint, IntPtr marshal_data) [0x00000] in <filename unknown>:0