Lock screen orientation when targeting Android API 27 with a non-opaque activity

I also faced the same problem. As others said, If I deleted android:screenOrientation="portrait" or overrided it with android:screenOrientation="unspecified", then the exception was gone. And it seems that the front activity's orientation follows the behind activity's orientation.

I thought about it. If the front activity is transparent and the behind activity's orientation is different, the display becomes strange. So, I can understand why this check logic was added However, I wonder that why this problem was not occurred in Developer Preview 8.0.0.

I solved this issues by changing this line in NoActionBar styles

In target version 27 only i got this issue and i solved by using below line

<item name="android:windowIsTranslucent">false</item>

The solution worked for me is deleting

android:screenOrientation="portrait" 

from all the full screen transparent activities which means their theme contains

<item name="android:windowIsTranslucent">true</item>

Also to make sure that orientation works correct for below Oreo I added this to the onCreate() of the activities.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // This activity is a fullscreen transparent activity, so after Oreo Android doesn't allow fullscreen
    // transparent activities to specify android:screenOrientation="portrait" in the manifest. It will pick up
    // from the background activity. But for below Oreo we should make sure that requested orientation is portrait.
    if (VERSION.SDK_INT < VERSION_CODES.O) {
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    }
}

The workaround is to set targetSdk back to 26.

The reason why is your application crashes is here in this commit.

As you can see here, you are not the only one - this behavior has been reported to Google as issue. It has been fixed, but we don't know how and when it will be released.

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I can also confirm what "sofakingforever" says in comments, if there is non-translucent activity with fixed orientation behind your translucent, the translucent will not rotate. So you can just remove android:screenOrientation="portrait" from manifest as well.