NullPointerException when a classpath folder does not exist

[AaronAsAChimp]

When running the checker with a classpath folder doesn't exist a NullPointerException is thrown. This is likely because calling File.list() (in CheckerMain.java:540) returns null if the directory does not exist.

Commands
The '*' is important.

javacheck -cp .\does-not-exists\*

Inputs
The exact input file doesn't matter, the checker doesn't progress far enough to check the source file (as seen above).

Outputs

Exception in thread "main" java.lang.NullPointerException
        at java.util.Objects.requireNonNull(Objects.java:203)
        at java.util.Arrays$ArrayList.<init>(Arrays.java:3813)
        at java.util.Arrays.asList(Arrays.java:3800)
        at org.checkerframework.framework.util.CheckerMain.jarFiles(CheckerMain.java:540)
        at org.checkerframework.framework.util.CheckerMain.expandWildcards(CheckerMain.java:524)
        at org.checkerframework.framework.util.CheckerMain.concatenatePaths(CheckerMain.java:507)
        at org.checkerframework.framework.util.CheckerMain.getExecArguments(CheckerMain.java:475)
        at org.checkerframework.framework.util.CheckerMain.invokeCompiler(CheckerMain.java:545)
        at org.checkerframework.framework.util.CheckerMain.main(CheckerMain.java:62)

Expectation
A NullPointerException does not occur and runs the checker. The javac compiler ignores classpath folders that do not exist.

[smillst]

Thanks for reporting. I can reproduced this and will fix it.