This breaks a lot of code that was working before

I've submitted similar report. You are welcome to vote, if you are affected.
https://issuetracker.unity3d.com/issues/android-application-crashes-when-native-function-returns-null

Hey @UnityTeam could you please provide info why the ticket was closed and marked "By design"?
How should we return "null" value from Java side if it keeps crashing?

Thanks in advance.

This issue still persists in 2018.2.1f1.

This issue breaks many 3rd party Assets that were previously working. I've received emails related to Assets "crashing" and it turns out to be this issue.

Surely this is not by design. Please reconsider and re-open this issue.

Please fix this issue and remove this wrongly attached "by design" label!

Workarounds might help for our own code, but not for all third-party libraries that access their native code... I will not really adjust them all.

I have made such workaround, but gosh... i hope this will be fixed in Unity

public static string safeCallStringMethod(AndroidJavaObject javaObject, string methodName, params object[] args)
{
#if UNITY_2018_2_OR_NEWER
if (args == null) args = new object[] { null };
IntPtr methodID = AndroidJNIHelper.GetMethodID<string>(javaObject.GetRawClass(), methodName, args, false);
jvalue[] jniArgs = AndroidJNIHelper.CreateJNIArgArray(args);

try
{
IntPtr returnValue = AndroidJNI.CallObjectMethod(javaObject.GetRawObject(), methodID, jniArgs);
if (IntPtr.Zero != returnValue)
{
var val = AndroidJNI.GetStringUTFChars(returnValue);
AndroidJNI.DeleteLocalRef(returnValue);
return val;
}
}
finally
{
AndroidJNIHelper.DeleteJNIArgArray(args, jniArgs);
}

return null;
#else
retun javaObject.Call<string>(methodName, args);
#endif
}

--java native code that--

public class CrashTest {

private static String testString = null;

public static String GetTestStr() {
return testString;
}

public static void SetTestStr(String str) {
testString = str;
}
}

pid: 19867, tid: 19893, name: UnityMain >>> com.test.MyApp <<<
r0 00000000 r1 00000000 r2 b2693880 r3 00000000
r4 e837e3a0 r5 00000000 r6 000033a2 r7 f30441a9
r8 c93e3d28 r9 e837edf4 sl e837ed08 fp e837e4b0
ip d3a0dcdc sp e837e348 lr d2943388 pc f49a3320 cpsr f625ec90
at libc.strlen(strlen:11)
at libunity.00122384(Native Method)

my crash logcat message

Is this by design?

I have no workaround for this problem.

Yes, more details, please! How some cryptic crash is part of this design?