RFR: JDK-8294567: IGV: IllegalStateException in search

[Tobias Holenstein]

When searching for a node, IGV first looks in the current open graph. If it can find the node here everything works fine. 

# Problem
If if cannot find the node if uses the `searchForward` and `searchBackward` in the `EditorInputGraphProvider` to search in the other graphs. It crashes here because `editor.isOpened()` can only be called from the `EventDispatchThread`

# Solution
The calls to `editor.isOpened()` are not needed because `editor != null` already means that it is open. So just remove all 4 calls to `editor.isOpened()`.

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Commit messages:
 - JDK-8294567: IGV: IllegalStateException in search

Changes: https://git.openjdk.org/jdk/pull/10483/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10483&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294567
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10483.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10483/head:pull/10483

PR: https://git.openjdk.org/jdk/pull/10483