RFR: 8187145: (Tree)TableView with null selectionModel: throws NPE on sorting [v7]

[Andy Goryachev]

> Setting a null selection model in TableView and TreeTableView produce NPE on sorting (and probably in some other situations) because the check for null is missing in several places. 
> 
> Setting a null selection model is a valid way to disable selection in a (tree)table.
> 
> There is also a similar issue with ListView [JDK-8279640](https://bugs.openjdk.org/browse/JDK-8279640).
> 
> changes:
> - added check for null selection model where it was missing
> - clear focused row index on setting null selection model in TreeTableView
> - clear selected cells on setting a new selection model

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  8187145: review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/876/files
  - new: https://git.openjdk.org/jfx/pull/876/files/efe84b7a..45b2e03a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=876&range=06
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=876&range=05-06

  Stats: 31 lines in 2 files changed: 26 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/876.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/876/head:pull/876

PR: https://git.openjdk.org/jfx/pull/876