NullReferenceException is thrown on double-clicking positons size index in the Line Renderer component.

Steps to repro:
1. Create a new project.
2. Gameobject > Creat Empty > Add Line Renderer Component.
3. Expand Positions property section in the inspector.
4. Double click on Positons size index.

Actual Result:
NullReferenceException is thrown in the console.

Expected Result:
No error occurs in the console.

Reproducible in:
2019.3.0a12, 2019.2.0f1, 2019.1.0f1, 2018.3.8f1.

Environment:
Windows & Mac 10.14.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.LineRendererPositionsView.DoubleClickedItem (System.Int32 id) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.IMGUI.Controls.TreeViewController.HandleUnusedMouseEventsForItem (UnityEngine.Rect rect, UnityEditor.IMGUI.Controls.TreeViewItem item, System.Int32 row) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.IMGUI.Controls.TreeViewController.DoItemGUI (UnityEditor.IMGUI.Controls.TreeViewItem item, System.Int32 row, System.Single rowWidth, System.Boolean hasFocus) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.IMGUI.Controls.TreeViewController.IterateVisibleItems (System.Int32 firstRow, System.Int32 numVisibleRows, System.Single rowWidth, System.Boolean hasFocus) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.IMGUI.Controls.TreeViewController.OnGUI (UnityEngine.Rect rect, System.Int32 keyboardControlID) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.IMGUI.Controls.TreeView.TreeViewWithMultiColumnHeader (UnityEngine.Rect rect) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.IMGUI.Controls.TreeView.OnGUI (UnityEngine.Rect rect) (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.LineRendererInspector.OnInspectorGUI () (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass53_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <2c7108252b3f412abf2ec8c94fb7c234>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)