fix: handle missing active theme config

build.gradle

@@ -22,6 +22,7 @@ dependencies {
 
     testImplementation 'run.halo.app:api'
     testImplementation 'org.springframework.boot:spring-boot-starter-test'
+    testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
 }
 
 test {

src/main/java/run/halo/live2d/ThemeFetcher.java

@@ -30,10 +30,12 @@ public Mono<String> getActiveThemeName() {
         return this.extensionClient.fetch(ConfigMap.class,
                 SystemSetting.SYSTEM_CONFIG
             )
-            .map(ConfigMap::getData)
-            .map(data -> JsonUtils.jsonToObject(
-                data.get("theme"), JsonNode.class).get("active").asText()
-            );
+            .flatMap(configMap -> Mono.justOrEmpty(configMap.getData()))
+            .flatMap(data -> Mono.justOrEmpty(data.get("theme")))
+            .filter(themeConfig -> !themeConfig.isBlank())
+            .map(themeConfig -> JsonUtils.jsonToObject(themeConfig, JsonNode.class))
+            .map(themeConfig -> themeConfig.path("active").asText(null))
+            .filter(themeName -> themeName != null && !themeName.isBlank());
     }
 
     public Mono<String> getActiveThemeLive2dTipsPath(String pathTemplate) {

src/test/java/run/halo/live2d/ThemeFetcherTest.java

@@ -0,0 +1,52 @@
+package run.halo.live2d;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Mono;
+import run.halo.app.extension.ConfigMap;
+import run.halo.app.extension.ReactiveExtensionClient;
+import run.halo.app.infra.SystemSetting;
+
+class ThemeFetcherTest {
+
+    private final ReactiveExtensionClient extensionClient = mock(ReactiveExtensionClient.class);
+
+    @Test
+    void shouldReturnEmptyWhenThemeConfigIsMissing() {
+        var configMap = new ConfigMap();
+        configMap.setData(Map.of());
+        when(extensionClient.fetch(ConfigMap.class, SystemSetting.SYSTEM_CONFIG))
+            .thenReturn(Mono.just(configMap));
+
+        var themeFetcher = new ThemeFetcher(extensionClient);
+
+        assertThat(themeFetcher.getActiveThemeName().blockOptional()).isEmpty();
+    }
+
+    @Test
+    void shouldReturnEmptyWhenThemeConfigDataIsNull() {
+        var configMap = new ConfigMap();
+        when(extensionClient.fetch(ConfigMap.class, SystemSetting.SYSTEM_CONFIG))
+            .thenReturn(Mono.just(configMap));
+
+        var themeFetcher = new ThemeFetcher(extensionClient);
+
+        assertThat(themeFetcher.getActiveThemeName().blockOptional()).isEmpty();
+    }
+
+    @Test
+    void shouldReturnActiveThemeName() {
+        var configMap = new ConfigMap();
+        configMap.setData(Map.of("theme", "{\"active\":\"default\"}"));
+        when(extensionClient.fetch(ConfigMap.class, SystemSetting.SYSTEM_CONFIG))
+            .thenReturn(Mono.just(configMap));
+
+        var themeFetcher = new ThemeFetcher(extensionClient);
+
+        assertThat(themeFetcher.getActiveThemeName().blockOptional()).contains("default");
+    }
+}