When two reactions are created and the Signal of the first reaction is replace with the signal of the second reaction then Exception appears in the console, refer attached video.

Steps to repro:
1. Create a new project.
2. Hierarchy > Create > Create Empty
3. Inspector > Add Component > Signal Receiver
4. Add two Reactions
5. Signal dropdown > Create a New Signal for First Reaction
6. Create a New Signal for the second Reaction and Replaced with the first one

Actual Result:
ArgumentException is thrown on creating and replacing Signal with Existed Signal

Expected Result:
New signal is not replaced at all with the signal of previously created Reaction as every Reaction has a unique signal

Note: Signal Receiver is supporting from Unity 2019.1 onwards

Occurring on:
2020.2.0a9, 2020.1.0b7, 2019.3.12f1, 2019.2.13f1, 2019.1.11f1

Package found version: 1.4.0-preview.3, 1.3.2, 1.0.0

Environment:
Windows 10 & macOS 10.14

Stack trace:
ArgumentException: SignalAsset already used.
UnityEngine.Timeline.SignalReceiver.ChangeSignalAtIndex (System.Int32 idx, UnityEngine.Timeline.SignalAsset newKey) (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Runtime/Events/Signals/SignalReceiver.cs:134)
UnityEditor.Timeline.Signals.SignalReceiverItem.UnityEditor.Timeline.Signals.ISignalAssetProvider.CreateNewSignalAsset (System.String path) (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Editor/Signals/TreeView/SignalReceiverItem.cs:170)
UnityEditor.Timeline.Signals.SignalUtility.DrawSignalNames (UnityEditor.Timeline.Signals.ISignalAssetProvider assetProvider, UnityEngine.Rect position, UnityEngine.GUIContent label, System.Boolean multipleValues) (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Editor/Signals/SignalUtility.cs:46)
UnityEditor.Timeline.Signals.SignalReceiverItem.DrawSignalNameColumn (UnityEngine.Rect rect, System.Single padding, UnityEngine.Timeline.SignalReceiver target, System.Int32 rowIdx) (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Editor/Signals/TreeView/SignalReceiverItem.cs:84)
UnityEditor.Timeline.Signals.SignalReceiverItem.Draw (UnityEngine.Rect rect, System.Int32 colIdx, System.Int32 rowIdx, System.Single padding, UnityEngine.Timeline.SignalReceiver target) (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Editor/Signals/TreeView/SignalReceiverItem.cs:61)
UnityEditor.Timeline.Signals.SignalReceiverTreeView.RowGUI (UnityEditor.IMGUI.Controls.TreeView+RowGUIArgs args) (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Editor/Signals/TreeView/SignalReceiverTreeView.cs:111)
UnityEditor.IMGUI.Controls.TreeView+TreeViewControlGUI.OnContentGUI (UnityEngine.Rect rect, System.Int32 row, UnityEditor.IMGUI.Controls.TreeViewItem item, System.String label, System.Boolean selected, System.Boolean focused, System.Boolean useBoldFont, System.Boolean isPinging) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.IMGUI.Controls.TreeViewGUI.DoItemGUI (UnityEngine.Rect rect, System.Int32 row, UnityEditor.IMGUI.Controls.TreeViewItem item, System.Boolean selected, System.Boolean focused, System.Boolean useBoldFont) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.IMGUI.Controls.TreeViewGUI.OnRowGUI (UnityEngine.Rect rowRect, UnityEditor.IMGUI.Controls.TreeViewItem item, System.Int32 row, System.Boolean selected, System.Boolean focused) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.IMGUI.Controls.TreeViewController.DoItemGUI (UnityEditor.IMGUI.Controls.TreeViewItem item, System.Int32 row, System.Single rowWidth, System.Boolean hasFocus) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.IMGUI.Controls.TreeViewController.IterateVisibleItems (System.Int32 firstRow, System.Int32 numVisibleRows, System.Single rowWidth, System.Boolean hasFocus) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.IMGUI.Controls.TreeViewController.OnGUI (UnityEngine.Rect rect, System.Int32 keyboardControlID) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.IMGUI.Controls.TreeView.TreeViewWithMultiColumnHeader (UnityEngine.Rect rect) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.IMGUI.Controls.TreeView.OnGUI (UnityEngine.Rect rect) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.Timeline.Signals.SignalReceiverTreeView.Draw () (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Editor/Signals/TreeView/SignalReceiverTreeView.cs:49)
UnityEditor.Timeline.Signals.SignalReceiverInspector.OnInspectorGUI () (at Library/PackageCache/com.unity.timeline@1.4.0-preview.3/Editor/Signals/SignalReceiverInspector.cs:51)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)