AddMissingMethodDeclarationFixCore makes unsafe call to getFunctionalInterfaceMethod() - NPE

QuickAssistProcessorUtil has the following code,calling `getFunctionalInterfaceMethod` but checking the result for null:

```
public static IMethodBinding getFunctionalMethodForMethodReference(MethodReference methodReference) {
ITypeBinding targetTypeBinding= ASTNodes.getTargetType(methodReference);
if (targetTypeBinding == null)
return null;

IMethodBinding functionalMethod= targetTypeBinding.getFunctionalInterfaceMethod();
if (functionalMethod != null && functionalMethod.isSynthetic()) {
functionalMethod= Bindings.findOverriddenMethodInType(functionalMethod.getDeclaringClass(), functionalMethod);
}
return functionalMethod;
}
```

However similar code in `AddMissingMethodDeclarationFixCore` does not proceed in a null-safe fashion:

```
ITypeBinding[] parameterTypes= methodBinding.getParameterTypes();
ITypeBinding[] typeArguments= methodBinding.getTypeArguments();
ITypeBinding[] parameterTypesFunctionalInterface= parameterTypes[index].getFunctionalInterfaceMethod().getParameterTypes();
ITypeBinding returnTypeBindingFunctionalInterface= parameterTypes[index].getFunctionalInterfaceMethod().getReturnType();

```