Avoid crash when resolving ICompilationUnit with unknown source version

When deciding whether to enable preview or not, check that source
version is known, otherwise fail back to latest known version.
Avoid using AST class which would trigger (undesired) exception in such
case)

Fixes #3929

Author: mickaelistria

org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/CompilationUnit.java

@@ -46,6 +46,7 @@
 import org.eclipse.jdt.internal.compiler.IProblemFactory;
 import org.eclipse.jdt.internal.compiler.SourceElementParser;
 import org.eclipse.jdt.internal.compiler.ast.CompilationUnitDeclaration;
+import org.eclipse.jdt.internal.compiler.classfmt.ClassFileConstants;
 import org.eclipse.jdt.internal.compiler.env.IElementInfo;
 import org.eclipse.jdt.internal.compiler.impl.CompilerOptions;
 import org.eclipse.jdt.internal.compiler.lookup.TypeConstants;
@@ -1583,7 +1584,7 @@ public Map<String, String> getCustomOptions() {
 			IJavaProject parentProject = getJavaProject();
 			Map<String, String> parentOptions = parentProject == null ? JavaCore.getOptions() : parentProject.getOptions(true);
 			if (JavaCore.ENABLED.equals(parentOptions.get(JavaCore.COMPILER_PB_ENABLE_PREVIEW_FEATURES)) &&
-				AST.newAST(parentOptions).apiLevel() < AST.getJLSLatest()) {
+				CompilerOptions.versionToJdkLevel(parentOptions.getOrDefault(JavaCore.COMPILER_SOURCE, JavaCore.latestSupportedJavaVersion())) < ClassFileConstants.getLatestJDKLevel()) {
 				// Disable preview features for older Java releases as it causes the compiler to fail later
 				if (customOptions != null) {
 					customOptions.put(JavaCore.COMPILER_PB_ENABLE_PREVIEW_FEATURES, JavaCore.DISABLED);