iOS:fix when obj_retain DoricContext cause crash

doric-iOS/Pod/Classes/Extension/DoricBridgeExtension.m

@@ -34,7 +34,7 @@ @implementation DoricBridgeExtension
 
 - (id)callNativeWithContextId:(NSString *)contextId module:(NSString *)module method:(NSString *)method callbackId:(NSString *)callbackId argument:(id)argument {
     __strong DoricContext *context = [[DoricContextManager instance] getContext:contextId];
-    if (context.destroyed) {
+    if (!context || context.destroyed) {
         return nil;
     }
     Class pluginClass = [self.registry acquireNativePlugin:module];