Calling Remove(key) on an unfolded SerializedDictionary visible in the Inspector causes a NullReferenceException if that dictionary only had 1 element.

[ClemGG]

This error does not happen when clicking on the '-' button in the Inspector, only via code and when the serialized dictionary has 1 element left.
Unity version 6000.0.32f1

Retrieving array element that was out of bounds UnityEditor.SerializedProperty:GetArrayElementAtIndex (int) AYellowpaper.SerializedCollections.Editor.States.DefaultListState:GetPropertyAtIndex (int) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/States/DefaultListState.cs:41) AYellowpaper.SerializedCollections.Editor.SerializedDictionaryInstanceDrawer:OnGetElementHeight (int) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/SerializedDictionaryInstanceDrawer.cs:607) UnityEditorInternal.ReorderableList:CacheIfNeeded () UnityEditorInternal.ReorderableList:GetListElementHeight () UnityEditorInternal.ReorderableList:GetHeight () AYellowpaper.SerializedCollections.Editor.SerializedDictionaryInstanceDrawer:GetPropertyHeight (UnityEngine.GUIContent) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/SerializedDictionaryInstanceDrawer.cs:118) AYellowpaper.SerializedCollections.Editor.SerializedDictionaryDrawer:GetPropertyHeight (UnityEditor.SerializedProperty,UnityEngine.GUIContent) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/SerializedDictionaryDrawer.cs:62)

NullReferenceException: Object reference not set to an instance of an object AYellowpaper.SerializedCollections.Editor.SerializedDictionaryInstanceDrawer.CalculateHeightOfElement (UnityEditor.SerializedProperty property, System.Boolean drawKeyAsList, System.Boolean drawValueAsList) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/SerializedDictionaryInstanceDrawer.cs:152) AYellowpaper.SerializedCollections.Editor.SerializedDictionaryInstanceDrawer.OnGetElementHeight (System.Int32 index) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/SerializedDictionaryInstanceDrawer.cs:608) UnityEditorInternal.ReorderableList.CacheIfNeeded () (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEditorInternal.ReorderableList.GetListElementHeight () (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEditorInternal.ReorderableList.GetHeight () (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) AYellowpaper.SerializedCollections.Editor.SerializedDictionaryInstanceDrawer.GetPropertyHeight (UnityEngine.GUIContent label) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/SerializedDictionaryInstanceDrawer.cs:118) AYellowpaper.SerializedCollections.Editor.SerializedDictionaryDrawer.GetPropertyHeight (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at Assets/Herve/Plugins/SerializedCollections/Editor/Scripts/SerializedDictionaryDrawer.cs:62)

ArgumentException: Getting control 0's position in a group with only 0 controls when doing repaint Aborting UnityEngine.GUILayoutGroup.GetNext () (at <44313a490f194ac399daed38f6ec3195>:0) UnityEngine.GUILayoutUtility.DoGetRect (System.Single minWidth, System.Single maxWidth, System.Single minHeight, System.Single maxHeight, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at <44313a490f194ac399daed38f6ec3195>:0) UnityEngine.GUILayoutUtility.GetRect (System.Single minWidth, System.Single maxWidth, System.Single minHeight, System.Single maxHeight, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at <44313a490f194ac399daed38f6ec3195>:0) UnityEditor.EditorGUILayout.GetControlRect (System.Boolean hasLabel, System.Single height, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEditor.EditorGUILayout.GetControlRect (System.Boolean hasLabel, System.Single height, UnityEngine.GUILayoutOption[] options) (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEditor.PropertyHandler.OnGUILayout (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEditor.UIElements.PropertyField.<CreatePropertyIMGUIContainer>b__51_0 () (at <878b6c863a9e4c42bf8483a7b6c60e0b>:0) UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.IMGUIContainer.DoIMGUIRepaint () (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.UIR.RenderChainCommand.ExecuteNonDrawMesh (UnityEngine.UIElements.UIR.DrawParams drawParams, System.Single pixelsPerPoint, System.Exception& immediateException) (at <743d97223c8b4d05a0ff33c535beb3d7>:0) Rethrow as ImmediateModeException UnityEngine.UIElements.UIR.RenderChain.Render () (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.UIRRepaintUpdater.Render () (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.BaseVisualElementPanel.Render () (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.Panel.Render () (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEditor.UIElements.EditorPanel.Render () (at <59f75a6879df4df1ac0158cb4ed845e6>:0) UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <743d97223c8b4d05a0ff33c535beb3d7>:0) UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <44313a490f194ac399daed38f6ec3195>:0)