I had similar issue under Ubuntu 20.04.6 LTS opening and pasting second new image.
Also had other Pinta crashes thats made me use GNU which is too much for my use.
I didn't have any crashes while running Ubuntu 18 that I can remember or at least not the frequency since I upgraded to Ubuntu 20.04 and Pinta 1.7.1.
So I finally was going to open a bug report but found this 2016 similar bug.
Do you need me to open a new bug report and what doc needs to be gathered?
Unhandled Execption:
Exception has been thrown by the target of an invocation.

Details
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.Gui.Widgets.CanvasRenderer.get_OffsetLayer () [0x00042] in <4b6bf512a4f54b1caeb747238f48b70c>:0 
  at Pinta.Gui.Widgets.CanvasRenderer.CreateOffsetLayer (Pinta.Core.Layer original) [0x00001] in <4b6bf512a4f54b1caeb747238f48b70c>:0 
  at Pinta.Gui.Widgets.CanvasRenderer.Render (System.Collections.Generic.List`1[T] layers, Cairo.ImageSurface dst, Gdk.Point offset) [0x000af] in <4b6bf512a4f54b1caeb747238f48b70c>:0 
  at Pinta.Gui.Widgets.LayersListWidget.Reset () [0x000d2] in <4b6bf512a4f54b1caeb747238f48b70c>:0 
  at Pinta.Gui.Widgets.LayersListWidget.HandleHistoryItemAdded (System.Object sender, System.EventArgs e) [0x00001] in <4b6bf512a4f54b1caeb747238f48b70c>:0 
  at (wrapper delegate-invoke) System.EventHandler`1[Pinta.Core.HistoryItemAddedEventArgs].invoke_void_object_TEventArgs(object,Pinta.Core.HistoryItemAddedEventArgs)
  at Pinta.Core.HistoryManager.OnHistoryItemAdded (Pinta.Core.BaseHistoryItem item) [0x00019] in <a4889b1da6f14fd68fb8fe10729e7f1d>:0 
  at Pinta.Core.DocumentWorkspaceHistory.PushNewItem (Pinta.Core.BaseHistoryItem newItem) [0x00117] in <a4889b1da6f14fd68fb8fe10729e7f1d>:0 
  at Pinta.Core.HistoryManager.PushNewItem (Pinta.Core.BaseHistoryItem newItem) [0x00010] in <a4889b1da6f14fd68fb8fe10729e7f1d>:0 
  at Pinta.Tools.MoveSelectedTool.OnFinishTransform (Cairo.Matrix transform) [0x00049] in <6b2549ee3b5d45fd8a1e7fd000a086f4>:0 
  at Pinta.Tools.BaseTransformTool.OnMouseUp (Gtk.DrawingArea canvas, Gtk.ButtonReleaseEventArgs args, Cairo.PointD point) [0x0001c] in <6b2549ee3b5d45fd8a1e7fd000a086f4>:0 
  at Pinta.Core.BaseTool.DoMouseUp (Gtk.DrawingArea canvas, Gtk.ButtonReleaseEventArgs args, Cairo.PointD point) [0x00030] in <a4889b1da6f14fd68fb8fe10729e7f1d>:0 
  at Pinta.Gui.Widgets.PintaCanvas+<PintaCanvas>c__AnonStorey0.<>m__3 (System.Object sender, Gtk.ButtonReleaseEventArgs e) [0x00038] in <4b6bf512a4f54b1caeb747238f48b70c>:0 
  at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo,object,object[],System.Exception&)
  at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0007c] in <12b418a7818c4ca0893feeaaf67f1e7f>:0 
   --- End of inner exception stack trace ---
  at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x0009a] in <12b418a7818c4ca0893feeaaf67f1e7f>:0 
  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <12b418a7818c4ca0893feeaaf67f1e7f>:0 
  at System.Delegate.DynamicInvokeImpl (System.Object[] args) [0x0010d] in <12b418a7818c4ca0893feeaaf67f1e7f>:0 
  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) [0x0000b] in <12b418a7818c4ca0893feeaaf67f1e7f>:0 
  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in <12b418a7818c4ca0893feeaaf67f1e7f>:0 
  at GLib.Signal.ClosureInvokedCB (System.Object o, GLib.ClosureInvokedArgs args) [0x0006d] in <918680a31aa84cb89cfa7cab56ea29b8>:0 
  at GLib.SignalClosure.Invoke (GLib.ClosureInvokedArgs args) [0x0000c] in <918680a31aa84cb89cfa7cab56ea29b8>:0 
  at GLib.SignalClosure.MarshalCallback (System.IntPtr raw_closure, System.IntPtr return_val, System.UInt32 n_param_vals, System.IntPtr param_values, System.IntPtr invocation_hint, System.IntPtr marshal_data) [0x0012f] in <918680a31aa84cb89cfa7cab56ea29b8>:0