FieldCanBeFinal - IllegalArgumentException (regression)

```
/Users/ben/projects/caffeine/caffeine/src/main/java/com/github/benmanes/caffeine/cache/LocalAsyncCache.java:16: error: An unhandled exception was thrown by the Error Prone static analysis plugin.
package com.github.benmanes.caffeine.cache;
^
Please report this at https://github.com/google/error-prone/issues/new and include the following:

error-prone version: 2.12.1
BugPattern: FieldCanBeFinal
Stack Trace:
java.lang.RuntimeException: java.lang.IllegalArgumentException: /Users/ben/projects/caffeine/caffeine/src/main/java/com/github/benmanes/caffeine/cache/LocalAsyncCache.java
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.handleExceptions(JavacTaskImpl.java:163)
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.analyze(JavacTaskImpl.java:370)
at com.google.errorprone.fixes.SuggestedFixes$FixCompiler.compile(SuggestedFixes.java:1372)
at com.google.errorprone.fixes.SuggestedFixes.compilesWithFix(SuggestedFixes.java:1300)
at com.google.errorprone.fixes.SuggestedFixes.compilesWithFix(SuggestedFixes.java:1260)
at com.google.errorprone.fixes.SuggestedFixes.compilesWithFix(SuggestedFixes.java:1242)
at com.google.errorprone.bugpatterns.FieldCanBeFinal.lambda$matchCompilationUnit$0(FieldCanBeFinal.java:238)
at java.base/java.util.Optional.ifPresent(Optional.java:183)
at com.google.errorprone.bugpatterns.FieldCanBeFinal.matchCompilationUnit(FieldCanBeFinal.java:236)
at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:449)
at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:555)
at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:591)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:56)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:58)
at com.google.errorprone.scanner.ErrorProneScannerTransformer.apply(ErrorProneScannerTransformer.java:43)
at com.google.errorprone.ErrorProneAnalyzer.finished(ErrorProneAnalyzer.java:152)
```