Fix a crash when instance is null

app/src/main/java/app/fedilab/android/BaseMainActivity.java

@@ -1114,7 +1114,7 @@ public abstract class BaseMainActivity extends BaseActivity implements NetworkSt
                         .proxy(Helper.getProxy(getApplication().getApplicationContext()))
                         .build();
                 Retrofit retrofit = new Retrofit.Builder()
-                        .baseUrl("https://" + IDN.toASCII(MainActivity.currentInstance, IDN.ALLOW_UNASSIGNED) + "/api/v1/")
+                        .baseUrl("https://" + (MainActivity.currentInstance != null ? IDN.toASCII(MainActivity.currentInstance, IDN.ALLOW_UNASSIGNED) : null) + "/api/v1/")
                         .addConverterFactory(GsonConverterFactory.create(Helper.getDateBuilder()))
                         .client(okHttpClient)
                         .build();

app/src/main/java/app/fedilab/android/mastodon/helper/TimelineHelper.java

@@ -62,7 +62,7 @@ public class TimelineHelper {
                 .proxy(Helper.getProxy(context))
                 .build();
         Retrofit retrofit = new Retrofit.Builder()
-                .baseUrl("https://" + IDN.toASCII(BaseMainActivity.currentInstance, IDN.ALLOW_UNASSIGNED) + "/api/v2/")
+                .baseUrl("https://" + (MainActivity.currentInstance != null ? IDN.toASCII(MainActivity.currentInstance, IDN.ALLOW_UNASSIGNED) : null) + "/api/v2/")
                 .addConverterFactory(GsonConverterFactory.create())
                 .client(okHttpClient)
                 .build();