Here's the UI error from 2020.2 Alex, it seems to happen for various reasons on my end and not just related to the output of actions although that does seem to trigger it more often. It also happens sometimes when entering a template and going back to the parent FSM in the editor.
System.ArgumentNullException: Value cannot be null.
Parameter name: key
at System.Collections.Generic.Dictionary`2[TKey,TValue].FindEntry (TKey key) [0x00008] in <9577ac7a62ef43179789031239ba8798>:0
at System.Collections.Generic.Dictionary`2[TKey,TValue].TryGetValue (TKey key, TValue& value) [0x00000] in <9577ac7a62ef43179789031239ba8798>:0
at HutongGames.PlayMaker.FsmEvent.GetFsmEvent (HutongGames.PlayMaker.FsmEvent fsmEvent) [0x00011] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Classes\FsmEvent.cs:475
at HutongGames.PlayMaker.FsmEventMapping.Init () [0x00011] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Classes\CustomActionTypes\FsmEventMapping.cs:29
at HutongGames.PlayMaker.FsmTemplateControl.InitOverrides () [0x001c6] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Classes\CustomActionTypes\FsmTemplateControl.cs:389
at HutongGames.PlayMaker.FsmTemplateControl.Init () [0x00009] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Classes\CustomActionTypes\FsmTemplateControl.cs:324
at FsmTemplateControlEditor.Init () [0x00000] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Classes\CustomActionTypes\Editor\FsmTemplateControlEditor.cs:65
at FsmTemplateControlEditor..ctor (HutongGames.PlayMaker.FsmTemplateControl fsmTemplateControl) [0x0002b] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Classes\CustomActionTypes\Editor\FsmTemplateControlEditor.cs:60
at FsmTemplateControlEditor.GetEditor (HutongGames.PlayMaker.FsmTemplateControl fsmTemplateControl) [0x0000f] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Classes\CustomActionTypes\Editor\FsmTemplateControlEditor.cs:40
at HutongGames.PlayMakerEditor.ActionEditor.EditFsmTemplateControl (System.Object fieldValue) [0x00010] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Editor\Classes\ActionEditor.cs:1966
at HutongGames.PlayMakerEditor.ActionEditor.GUIForFieldTypes (System.String labelText, System.Type type, System.Object fieldValue, System.Object[] attributes) [0x00194] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Editor\Classes\ActionEditor.cs:979
at HutongGames.PlayMakerEditor.ActionEditor.EditField (System.Object obj, System.Reflection.FieldInfo field, System.String labelText, System.Type fieldType, System.Object fieldValue, System.Object[] attributes) [0x0000c] in C:\Projects\Playmaker_1.9.1\Projects\Playmaker.source.unity\Assets\PlayMaker\Editor\Classes\ActionEditor.cs:620
Unity 2020.2.1f1
PM 1.9.1.f7