Fix question condition validation in survey editor.

frontend/src/components/stages/survey_editor.ts

@@ -15,6 +15,7 @@ import {
   getConditionTargetsFromStages,
   MultipleChoiceItem,
   MultipleChoiceSurveyQuestion,
+  sanitizeSurveyQuestionConditions,
   ScaleSurveyQuestion,
   SurveyPerParticipantStageConfig,
   SurveyStageConfig,
@@ -103,36 +104,47 @@ export class SurveyEditor extends MobxLitElement {
     }
   }
 
+  /**
+   * Update the stage with new questions, automatically sanitizing conditions.
+   */
+  private updateStageQuestions(questions: SurveyQuestion[]) {
+    if (!this.stage) return;
+
+    const sanitizedQuestions = sanitizeSurveyQuestionConditions(
+      questions,
+      this.stage.id,
+    );
+
+    this.experimentEditor.updateStage({
+      ...this.stage,
+      questions: sanitizedQuestions,
+    });
+  }
+
   moveQuestionUp(index: number) {
     if (!this.stage) return;
 
     const questions = [
       ...this.stage.questions.slice(0, index - 1),
-      ...this.stage.questions.slice(index, index + 1),
-      ...this.stage.questions.slice(index - 1, index),
+      this.stage.questions[index],
+      this.stage.questions[index - 1],
       ...this.stage.questions.slice(index + 1),
     ];
 
-    this.experimentEditor.updateStage({
-      ...this.stage,
-      questions,
-    });
+    this.updateStageQuestions(questions);
   }
 
   moveQuestionDown(index: number) {
     if (!this.stage) return;
 
     const questions = [
       ...this.stage.questions.slice(0, index),
-      ...this.stage.questions.slice(index + 1, index + 2),
-      ...this.stage.questions.slice(index, index + 1),
+      this.stage.questions[index + 1],
+      this.stage.questions[index],
       ...this.stage.questions.slice(index + 2),
     ];
 
-    this.experimentEditor.updateStage({
-      ...this.stage,
-      questions,
-    });
+    this.updateStageQuestions(questions);
   }
 
   deleteQuestion(index: number) {
@@ -143,10 +155,7 @@ export class SurveyEditor extends MobxLitElement {
       ...this.stage.questions.slice(index + 1),
     ];
 
-    this.experimentEditor.updateStage({
-      ...this.stage,
-      questions,
-    });
+    this.updateStageQuestions(questions);
   }
 
   updateQuestion(question: SurveyQuestion, index: number) {

utils/src/utils/condition.utils.ts

@@ -10,6 +10,7 @@ import {
   ConditionTargetReference,
   getConditionTargetKey,
   extractMultipleConditionDependencies,
+  extractConditionDependencies,
   evaluateCondition,
   Condition,
 } from './condition';
@@ -311,3 +312,50 @@ export function getConditionTargetsFromStages(
 
   return targets;
 }
+
+// ============================================================================
+// Condition Sanitization Utilities
+// ============================================================================
+
+/**
+ * Sanitize survey question conditions to ensure they only reference valid targets.
+ *
+ * A condition is invalid if it references:
+ * - A question that doesn't exist in the list
+ * - A question that comes at or after the current question's position
+ *
+ * Invalid conditions are cleared (set to undefined) to prevent rendering issues.
+ *
+ * @param questions - The list of survey questions to sanitize
+ * @param stageId - The ID of the stage containing these questions
+ * @returns A new array with invalid conditions cleared
+ */
+export function sanitizeSurveyQuestionConditions<
+  T extends {id: string; condition?: Condition},
+>(questions: T[], stageId: string): T[] {
+  // Build a map of question ID to its index in the ordering
+  const questionIndexMap = new Map<string, number>();
+  questions.forEach((q, idx) => {
+    questionIndexMap.set(q.id, idx);
+  });
+
+  return questions.map((question, index) => {
+    if (!question.condition) return question;
+
+    const dependencies = extractConditionDependencies(question.condition);
+
+    // Check if any dependency in the same stage is invalid
+    const hasInvalidDependency = dependencies.some((dep) => {
+      if (dep.stageId !== stageId) return false; // Other stages are fine
+
+      const refIndex = questionIndexMap.get(dep.questionId);
+      // Invalid if: question doesn't exist, or comes at/after current position
+      return refIndex === undefined || refIndex >= index;
+    });
+
+    if (hasInvalidDependency) {
+      return {...question, condition: undefined};
+    }
+    return question;
+  });
+}