JDeveloper hangs while using Expression Builder in Assign

Using the Expression Builder to build XPath expressions may cause Oracle JDeveloper to hang. If that happens, perform the following steps:
1. Kill the Oracle JDeveloper process.
2. Restart Oracle JDeveloper.
3. Select Tools > Preferences > SOA, and deselect the Validate Expression checkbox.
After performing these steps, Oracle JDeveloper should no longer hang.