Reword error message

Co-Authored-By: Yossi Farjoun <[email protected]>

src/main/java/htsjdk/variant/variantcontext/VariantContext.java

@@ -394,7 +394,7 @@ private static void validateFilters(final VariantContext variantContext) {
 
             for (String filter : filters) {
                 if ( filter == null) {
-                    throw new IllegalStateException("'null' is not a valid filter string.");
+                    throw new IllegalArgumentException("Filter string cannot be null.");
                 }
                 if (!VALID_FILTER.matcher(filter).matches()) {
                     throw new IllegalStateException("Filter '" + filter +