errors when disabling multi select support

[jrieken]

re #64727

• in joh/o2
• disable multi select support via multipleSelectionSupport
• select an element, the error below happens

ERR Cannot read property 'isSelectionRangeChangeEvent' of undefined: TypeError: Cannot read property 'isSelectionRangeChangeEvent' of undefined
    at TreeNodeListMouseController.onPointer (file:///Users/jrieken/Code/vscode/out/vs/base/browser/ui/tree/abstractTree.js:634:50)
    at file:///Users/jrieken/Code/vscode/out/vs/base/common/event.js:94:61
    at Emitter.fire (file:///Users/jrieken/Code/vscode/out/vs/base/common/event.js:542:38)
    at HTMLDivElement.fn (file:///Users/jrieken/Code/vscode/out/vs/base/browser/event.js:9:48)