[voice_assistant][micro_wake_word] Fix null deref and missing error return (#14906)

esphome/components/micro_wake_word/streaming_model.cpp

@@ -80,6 +80,7 @@ bool StreamingModel::load_model_() {
     TfLiteTensor *output = this->interpreter_->output(0);
     if ((output->dims->size != 2) || (output->dims->data[0] != 1) || (output->dims->data[1] != 1)) {
       ESP_LOGE(TAG, "Streaming model tensor output dimension is not 1x1.");
+      return false;
     }
 
     if (output->type != kTfLiteUInt8) {

esphome/components/voice_assistant/voice_assistant.cpp

@@ -619,6 +619,8 @@ void VoiceAssistant::start_playback_timeout_() {
     this->cancel_timeout("speaker-timeout");
     this->set_state_(State::RESPONSE_FINISHED, State::RESPONSE_FINISHED);
 
+    if (this->api_client_ == nullptr)
+      return;
     api::VoiceAssistantAnnounceFinished msg;
     msg.success = true;
     this->api_client_->send_message(msg);