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

[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.

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Commit messages:
 - Prevent OS crash when contentView is set to nil while processing shortcut

Changes: https://git.openjdk.java.net/jfx/pull/714/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=714&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242544
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 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