RFR: 8266539: [TreeView]: Change.getRemoved() contains null item when deselecting a TreeItem

[Michael Strauß]

This PR contains a fix that was de-scoped from #480 and solves an issue where an incorrect index is used in `MultipleSelectionModelBase.SelectedIndicesList.clear(int)`.

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Commit messages:
 - Fixed incorrect index
 - Failing test

Changes: https://git.openjdk.java.net/jfx/pull/492/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=492&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266539
  Stats: 17 lines in 2 files changed: 14 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/492.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/492/head:pull/492

PR: https://git.openjdk.java.net/jfx/pull/492