Check for null list. Crash fix.

pom.xml

@@ -63,7 +63,7 @@ http://maven.apache.org/maven-v4_0_0.xsd">
 					<skipTests>true</skipTests>
 				</configuration>
 			</plugin>
-			<plugin>
+			<!-- plugin>
 		        <groupId>org.apache.maven.plugins</groupId>
 		        <artifactId>maven-compiler-plugin</artifactId>
 		        <version>3.8.1</version>
@@ -75,7 +75,7 @@ http://maven.apache.org/maven-v4_0_0.xsd">
 		          <source>1.8</source>
 		          <target>1.8</target>
 		        </configuration>
-		    </plugin>	
+		    </plugin -->	
 			<plugin>
 				<groupId>org.antlr</groupId>
 				<artifactId>antlr4-maven-plugin</artifactId>

src/main/java/de/dhbwstuttgart/typeinference/unify/TypeUnifyTask.java

@@ -1224,7 +1224,7 @@ public class TypeUnifyTask extends RecursiveTask<Set<Set<UnifyPair>>>  {
 						if (!isUndefinedPairSetSet(par_res) && isUndefinedPairSetSet(result)) {
 							//wenn korrektes Ergebnis gefunden alle Fehlerfaelle loeschen
 							result = par_res;
-							if (par_res.iterator().next() instanceof WildcardType) {
+							if (!par_res.isEmpty() && par_res.iterator().next() instanceof WildcardType) {
 								System.out.println("");
 							}
 						}