A DESCRIPTION OF THE PROBLEM :
In this case, the core modules are built with Java 15 (build 15-ea+34-1554), bytecode compatibility 15 (Bytecode version 59) and enabled preview feature . The code is actually "Java 14+ Preview-Feature" compatible (at least bytecode version 58.65535).
However, the JavaFX GUI requires Java 14 (build 14.0.2+12-46), so it cannot handle the 15-byte code of the core classes: major version 59 is newer than 58, the highest major version supported by this compiler.

When compiling the GUI sources and modules, which contains references to the core modules (Bytecode version 59), with Java 14 an exception occurred.
[javac] java.lang.NullPointerException
[javac] at jdk.compiler/com.sun.tools.javac.code.Preview.warnPreview(Preview.java:146)
[javac] at jdk.compiler/com.sun.tools.javac.jvm.ClassReader.readClassBuffer(ClassReader.java:2559)
[javac] at jdk.compiler/com.sun.tools.javac.jvm.ClassReader.readClassFile(ClassReader.java:2583)
[javac] at jdk.compiler/com.sun.tools.javac.code.ClassFinder.fillIn(ClassFinder.java:362)
[javac] at jdk.compiler/com.sun.tools.javac.code.ClassFinder.complete(ClassFinder.java:292)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:670)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1383)
[javac] at jdk.compiler/com.sun.tools.javac.code.ClassFinder.loadClass(ClassFinder.java:435)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Resolve.loadClass(Resolve.java:2007)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Resolve.findIdentInPackageInternal(Resolve.java:2402)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Resolve.findIdentInPackage(Resolve.java:2390)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.selectSym(Attr.java:4184)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.visitSelect(Attr.java:4069)
[javac] at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:2276)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:663)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.attribType(Attr.java:729)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.attribType(Attr.java:722)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.attribImportType(TypeEnter.java:444)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.doImport(TypeEnter.java:427)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.resolveImports(TypeEnter.java:367)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.runPhase(TypeEnter.java:324)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$Phase.doCompleteEnvs(TypeEnter.java:285)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$Phase.completeEnvs(TypeEnter.java:254)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter.complete(TypeEnter.java:201)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:670)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1383)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Enter.complete(Enter.java:584)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Enter.main(Enter.java:561)
[javac] at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.enterTrees(JavaCompiler.java:1071)
[javac] at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:936)
[javac] at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:316)
[javac] at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
[javac] at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:57)
[javac] at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:43)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
javac
-d <destination directory for class files>
-classpath <cp>
-sourcepath <source code path JAVA 14 compatible>
-g
--enable-preview
--release 14

ACTUAL -
[javac] java.lang.NullPointerException
[javac] at jdk.compiler/com.sun.tools.javac.code.Preview.warnPreview(Preview.java:146)
[javac] at jdk.compiler/com.sun.tools.javac.jvm.ClassReader.readClassBuffer(ClassReader.java:2559)
[javac] at jdk.compiler/com.sun.tools.javac.jvm.ClassReader.readClassFile(ClassReader.java:2583)
[javac] at jdk.compiler/com.sun.tools.javac.code.ClassFinder.fillIn(ClassFinder.java:362)
[javac] at jdk.compiler/com.sun.tools.javac.code.ClassFinder.complete(ClassFinder.java:292)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:670)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1383)
[javac] at jdk.compiler/com.sun.tools.javac.code.ClassFinder.loadClass(ClassFinder.java:435)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Resolve.loadClass(Resolve.java:2007)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Resolve.findIdentInPackageInternal(Resolve.java:2402)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Resolve.findIdentInPackage(Resolve.java:2390)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.selectSym(Attr.java:4184)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.visitSelect(Attr.java:4069)
[javac] at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:2276)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.attribTree(Attr.java:663)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.attribType(Attr.java:729)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Attr.attribType(Attr.java:722)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.attribImportType(TypeEnter.java:444)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.doImport(TypeEnter.java:427)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.resolveImports(TypeEnter.java:367)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$ImportsPhase.runPhase(TypeEnter.java:324)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$Phase.doCompleteEnvs(TypeEnter.java:285)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter$Phase.completeEnvs(TypeEnter.java:254)
[javac] at jdk.compiler/com.sun.tools.javac.comp.TypeEnter.complete(TypeEnter.java:201)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol.complete(Symbol.java:670)
[javac] at jdk.compiler/com.sun.tools.javac.code.Symbol$ClassSymbol.complete(Symbol.java:1383)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Enter.complete(Enter.java:584)
[javac] at jdk.compiler/com.sun.tools.javac.comp.Enter.main(Enter.java:561)
[javac] at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.enterTrees(JavaCompiler.java:1071)
[javac] at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:936)
[javac] at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:316)
[javac] at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
[javac] at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:57)
[javac] at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:43)

FREQUENCY : always