From the PR body:

There is a method applyEditorTo in the JTRegConfigurable.java class, which is called repeatedly while the "Run Configuration" window is open. Within this method, the method jrePathEditor.getJrePathOrName() is invoked. As a result, when this method is called for a JRE path not listed in IDEA, it attempts to determine the Java version. If the user has selected a raw JDK image, a Java process is launched, and the version is obtained from the process output (see org.jetbrains.jps.model.java.impl.JdkVersionDetectorImpl#detectJdkVersionInfo).

This call is too resource-intensive to be executed on the UI thread, causing the window to freeze.