NPE while invoke toJson() or fromJson() for HashMap<String, String>

### the exception:
```
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.Object.hashCode()' on a null object reference
libcore.reflect.TypeVariableImpl.hashCode(TypeVariableImpl.java:47)
java.util.HashMap.hash(HashMap.java:338)
java.util.HashMap.containsKey(HashMap.java:595)
java.util.HashSet.contains(HashSet.java:203)
com.google.gson.internal.$Gson$Types.resolve(.java:346)
com.google.gson.internal.$Gson$Types.resolve(.java:381)
com.google.gson.internal.$Gson$Types.resolve(.java:337)
com.google.gson.internal.$Gson$Types.getSupertype(.java:283)
com.google.gson.internal.$Gson$Types.getCollectionElementType(.java:302)
com.google.gson.internal.bind.CollectionTypeAdapterFactory.create(CollectionTypeAdapterFactory.java:52)
com.google.gson.Gson.getAdapter(Gson.java:458)
com.google.gson.Gson.toJson(Gson.java:696)
com.google.gson.Gson.toJson(Gson.java:683)
com.google.gson.Gson.toJson(Gson.java:638)
com.google.gson.Gson.toJson(Gson.java:618)
```

### my code:

```java
new Gson().fromJson(json, new TypeToken>() {}.getType());
```

### environment

Gson version 2.8.5
The probability of this problem happens substantial increases，when gson version upgrades from 2.8.2 to 2.8.5 !

Same problem with https://github.com/google/gson/issues/1298 , but not resolved.