Getting rid of the "Allocating an object of abstract class type" error

Question

I'm trying to write a simple program and after adding some code, I stumble upon this guy:

juce::AudioProcessorEditor* BluePillAudioProcessor::createEditor()
{
    return new BluePillAudioProcessorEditor (*this);
}

It shows an error like in the title:

Allocating an object of abstract class type 'BluePillAudioProcessorEditor'

...apparently my class "BluePillAudioProcessorEditor" is abstract? I tried modifying so much of the code and yet it doesn't look good. This one line is why I wasted my entire evening. I admit that I'm a beginner in writing in C++.

Here's the definition of that class:

class BluePillAudioProcessorEditor  : public juce::AudioProcessorEditor,
            public juce::FileDragAndDropTarget,
            public juce::Button::Listener