RFR: 8242544: CMD+ENTER key event crashes the application when invoked on dialog [v2]

[Martin Fox]

> The OS crashes if the contentView of a window is set to nil while handling processKeyEquivalent. With this PR we just set the contentView to a basic do-nothing NSView for the interim.

Martin Fox has updated the pull request incrementally with one additional commit since the last revision:

  Releasing dummy NSView

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/714/files
  - new: https://git.openjdk.java.net/jfx/pull/714/files/5674733b..f71d80f8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=714&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=714&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/714.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/714/head:pull/714

PR: https://git.openjdk.java.net/jfx/pull/714