Two-value style slider error, using Slider

I am trying to make a basic synthesiser plugin. I have a problem with my Slider. When I connect it with a ValueTree, I SOMETIMES get the following error. Sometimes it works without error.

 // for a two-value style slider, you should use the setMinValue() and setMaxValue()
 // methods to set the two values.
 jassert (style != TwoValueHorizontal && style != TwoValueVertical);

I can’t explain why I’m getting a error about a two-value-slider, even though im not using one. Am I missing something?
I am new to JUCE and to C++ in general so please excuse me if I missed something obvious.

This is how I add the Parameter in PluginProcessor.cpp:

tree (*this, nullptr)

tree.createAndAddParameter("osc1_attack", "Osc1_Attack", "Osc1_Attack", envParam, 0, nullptr ,nullptr);

//Initialize Tree 
tree.state = ValueTree("SynthTree");

This is how I create the Slider in PluginEditor.cpp:

osc1AttackSlider.setRange(0.1f, 500.0f);
osc1AttackSlider.setTextBoxStyle(Slider::TextBoxBelow, false, 50, 20);

sliderTree = new AudioProcessorValueTreeState::SliderAttachment(processor.tree, "osc1_attack", osc1AttackSlider);

This is how I declared the sliderTree variable in PluginEditor.h:

AudioProcessorValueTreeState::SliderAttachment* sliderTree;

I followed the very good YouTube Tutorial of “theAudioProgrammer” and he used a ScopedPointer for this. For me that only works for ComboBoxes, if I use it here I get the following error:

/** Returns a raw pointer to the allocated data.
    This may be a null pointer if the data hasn't yet been allocated, or if it has been
    freed by calling the free() method.
inline ElementType* get() const noexcept                                 { return data; }