Description of problem:
Pinta throws an unhandled exception using the Help menu -> Pinta Website.

Version-Release number of selected component (if applicable):
pinta-1.6-16.fc32.x86_64

How reproducible:
Always.

Steps to Reproduce:
1. Launch Pinta
2. Select Help->Pinta Website
3.

Actual results:
Unhandled exception:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ComponentModel.Win32Exception: Cannot find the specified file
  at System.Diagnostics.Process.StartWithShellExecuteEx (System.Diagnostics.ProcessStartInfo startInfo) [0x00129] in <13e87ee1a85d4b5e8544328288bab252>:0 
  at System.Diagnostics.Process.Start () [0x00038] in <13e87ee1a85d4b5e8544328288bab252>:0 
  at (wrapper remoting-invoke-with-check) System.Diagnostics.Process.Start()
  at System.Diagnostics.Process.Start (System.Diagnostics.ProcessStartInfo startInfo) [0x0001e] in <13e87ee1a85d4b5e8544328288bab252>:0 
  at System.Diagnostics.Process.Start (System.String fileName) [0x00006] in <13e87ee1a85d4b5e8544328288bab252>:0 
  at Pinta.Core.HelpActions.Website_Activated (System.Object sender, System.EventArgs e) [0x00001] in <ee92d54e75164822a3385c045bf52e3a>: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 <51c7842c36134e179a4bb23e01c71109>: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 <51c7842c36134e179a4bb23e01c71109>:0 
  at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <51c7842c36134e179a4bb23e01c71109>:0 
  at System.Delegate.DynamicInvokeImpl (System.Object[] args) [0x0010d] in <51c7842c36134e179a4bb23e01c71109>:0 
  at System.MulticastDelegate.DynamicInvokeImpl (System.Object[] args) [0x0000b] in <51c7842c36134e179a4bb23e01c71109>:0 
  at System.Delegate.DynamicInvoke (System.Object[] args) [0x00000] in <51c7842c36134e179a4bb23e01c71109>:0 
  at GLib.Signal.ClosureInvokedCB (System.Object o, GLib.ClosureInvokedArgs args) [0x0007d] in <ed39f21b9e9343dcbd442a17ad356a9f>:0 
  at GLib.Signal+SignalClosure.Invoke (GLib.ClosureInvokedArgs args) [0x0000c] in <ed39f21b9e9343dcbd442a17ad356a9f>:0 
  at GLib.Signal+SignalClosure.MarshalCallback (System.IntPtr raw_closure, GLib.Value* return_val, System.UInt32 n_param_vals, GLib.Value* param_values, System.IntPtr invocation_hint, System.IntPtr marshal_data) [0x00050] in <ed39f21b9e9343dcbd442a17ad356a9f>:0 

Expected results:
Should launch default browser, which for me is Firefox.

Additional info:
I have confirmed Firefox is the default browser.