Fix crash in HeadlessAppLoader after the activity is killed

devSupportManager can be null. This is evident both from my experience and from similar code in:
https://github.com/expo/expo/blob/0ec9e78c4e4ff0656eaa861b50618a80b8650952/android/expoview/src/main/java/host/exp/exponent/experience/ReactNativeActivity.java#L293

This crash was preventing background fetch from working, as the app was crashing when trying to restart.
This is probably related to the many background task issues in the issue tracker such as: expo#3503, expo#3582, expo#4860 and
potentially others.

I have not been able to test it locally (no idea how to build expo kit, and wasn't able to get help on the Slack), though
the code seems right, and it's a trivial change.

android/expoview/src/main/java/host/exp/exponent/headless/HeadlessAppLoader.java

@@ -317,9 +317,12 @@ private RNObject startReactInstance(final Exponent.StartReactInstanceDelegate de
     }
 
     RNObject reactInstanceManager = builder.callRecursive("build");
-    RNObject devSettings = reactInstanceManager.callRecursive("getDevSupportManager").callRecursive("getDevSettings");
-    if (devSettings != null) {
-      devSettings.setField("exponentActivityId", mActivityId);
+    RNObject devSupportManager = mReactInstanceManager.callRecursive("getDevSupportManager");
+    if (devSupportManager != null) {
+      RNObject devSettings = devSupportManager.callRecursive("getDevSettings");
+      if (devSettings != null) {
+        devSettings.setField("exponentActivityId", mActivityId);
+      }
     }
 
     reactInstanceManager.call("createReactContextInBackground");