Improved handling of (potentially) null fields

runtime/Java/src/org/antlr/v4/runtime/DefaultErrorStrategy.java

@@ -126,7 +126,8 @@ public class DefaultErrorStrategy implements ANTLRErrorStrategy {
 //						   lastErrorIndex+
 //						   ", states="+lastErrorStates);
 		if ( lastErrorIndex==recognizer.getInputStream().index() &&
-		lastErrorStates.contains(recognizer._ctx.s) ) {
+			lastErrorStates != null &&
+			lastErrorStates.contains(recognizer._ctx.s) ) {
 			// uh oh, another error at same token index and previously-visited
 			// state in ATN; must be a case where LT(1) is in the recovery
 			// token set so nothing got consumed. Consume a single token

runtime/Java/src/org/antlr/v4/runtime/Lexer.java

@@ -130,6 +130,10 @@ public abstract class Lexer extends Recognizer<Integer, LexerATNSimulator>
 	 */
 	@Override
 	public Token nextToken() {
+		if (_input == null) {
+			throw new IllegalStateException("nextToken requires a non-null input stream.");
+		}
+
 		if (_hitEOF) {
 			emitEOF();
 			return _token;