If any of the heaps is empty, calling top() on them is undefined behavior. Consider changing the code like this

if (minHeap.empty()) {
   return maxHeap.empty() ? 0 : maxHeap.top();
}

if (maxHeap.size() == minHeap.size()) {
   return maxHeap.top() / 2.0 + minHeap.top() / 2.0;
}

return maxHeap.top();
}