WrongTimberUsageDetector.checkNestedStringFormat NPE

Hi! We use latest Timber version(4.7.1). I found some issue in custom Lint rule(WrongTimberUsageDetector). Did not go deeper to find reason, why it is crashing. Just post you simple code, that initiates NPE.

_Code:_
`TextUtils.join(",", Arrays.asList(String.format("Hello %s", "World"), "Hello World"));`

_Crash log:_
```
NullPointerException:
WrongTimberUsageDetector.checkNestedStringFormat(WrongTimberUsageDetector.java:111)?
WrongTimberUsageDetector.visitMethod(WrongTimberUsageDetector.java:74)?
UElementVisitor$DelegatingPsiVisitor.visitMethodCallExpression(UElementVisitor.kt:1027)?UElementVisitor$DelegatingPsiVisitor.visitCallExpression(UElementVisitor.kt:1007)?
UCallExpression$DefaultImpls.accept(UCallExpression.kt:99)?
JavaUCallExpression.accept(javaUCallExpressions.kt:24)?
UQualifiedReferenceExpression$DefaultImpls.accept(UQualifiedReferenceExpression.kt:48)?
JavaUCompositeQualifiedExpression.accept(JavaUCompositeQualifiedExpression.kt:22)
```

Hope, this info will be helpful for you to find solve,