Make the null-result case visible to the user

Fixes #1303.

(cherry picked from commit 83438644dcdc470517f67c0d55ff4889f71cc435)

worldedit-core/src/main/java/com/sk89q/worldedit/internal/expression/Expression.java

@@ -34,8 +34,6 @@ import java.time.Instant;
 import java.util.List;
 import java.util.Objects;
 
-import static com.google.common.base.Preconditions.checkNotNull;
-
 /**
  * Compiles and evaluates expressions.
  *
@@ -122,7 +120,9 @@ public class Expression {
         Instant deadline = Instant.now().plusMillis(timeout);
         // evaluation exceptions are thrown out of this method
         Double result = compiledExpression.execute(new ExecutionData(slots, functions, deadline));
-        checkNotNull(result, "Expression must result in a value");
+        if (result == null) {
+            throw new EvaluationException(-1, "Expression must result in a value");
+        }
         return result;
     }