Button and Input created with empty constructor crash page

[lemniscate8]

The Bug
Creating a button using the empty constructor is allowed but crashes a page.

To Reproduce
Here's some code:

#include "emp/web/web.hpp"
#include "emp/web/Button.hpp"

emp::web::Document doc("emp_base");
emp::web::Button button1([]() { 
  std::cout << "I don't work anymore!"; 
  }, "button1");
emp::web::Button button2; // He he, going to break everything
emp::web::Button button3([]() { 
  std::cout << "I don't even load. (Can't click meeeee!)"; 
  }, "button3");

int main() {
  doc << button1;
  doc << button2; // Does not render and breaks everything after it
  doc << button3;
}

Expected behavior
Shouldn't break everything or should generate a compiler error. But buttons without a callback would be useful. Maybe refactor to require an href at least and disable empty constructor?

Toolchain:

• OS: Mac
• Compiler: Emscripten 1.38.48
• Browser Chrome
• Empirical Version [e.g. the current master version, or a commit id]

Additional context
Gives the following error in console:
Uncaught abort(15). Build with -s ASSERTIONS=1 for more info.