优化

app/src/main/java/io/legado/app/data/entities/BaseBook.kt

@@ -27,8 +27,8 @@ interface BaseBook : RuleDataInterface {
         putVariable("custom", value)
     }
 
-    fun getCustomVariable(): String? {
-        return getVariable("custom")
+    fun getCustomVariable(): String {
+        return getVariable("custom") ?: ""
     }
 
     override fun putBigVariable(key: String, value: String?) {

app/src/main/java/io/legado/app/data/entities/BaseSource.kt

@@ -196,8 +196,8 @@ interface BaseSource : JsExtensions {
     /**
      * 获取自定义变量
      */
-    fun getVariable(): String? {
-        return CacheManager.get("sourceVariable_${getKey()}")
+    fun getVariable(): String {
+        return CacheManager.get("sourceVariable_${getKey()}") ?: ""
     }
 
     /**

app/src/main/java/io/legado/app/model/analyzeRule/RuleDataInterface.kt

@@ -24,8 +24,8 @@ interface RuleDataInterface {
 
     fun putBigVariable(key: String, value: String?)
 
-    fun getVariable(key: String): String? {
-        return variableMap[key] ?: getBigVariable(key)
+    fun getVariable(key: String): String {
+        return variableMap[key] ?: getBigVariable(key) ?: ""
     }
 
     fun getBigVariable(key: String): String?